Skip to main content

Full text of "Batteries Gates Cyclon Battery Application Manual OCR"

See other formats

Professional Program Session Record 

Battery Backup for the 
Life of the Product 


TELEPHONE (301) 296-7000 • TELEX. 87-768 

Session 17 

Battery Backup For the Life of the Product 

Session Organizer 
Edward S. Hatch, Jr. 
Catalyst Research Corp. 
Baltimore, MD 

Session Chairman 
Dr. Alan A. Schneider 
Catalyst Research Corp. 
Baltimore, MD 





Battery Backup For the Life of the Product 

Dr. Alan A. Schneider 
Catalyst Research Corp. 
Baltimore, MD 

Long Life Primary and Secondary Batteries 

Thomas Irwin 
Madison, WI 

Ultra-Low Power CMOS Memory for Battery Backup 

Dr. Isamu Kuru, Msayoshi Nakane, and Kaoru Tokushige 
Toshiba America, Inc. 
Irvine, CA 

Applications of Lithium-Iodine Batteries to CMOS 
Memory and Microprocessors 

John L. Stegman Jr. 
Catalyst Research Corp. 
Baltimore, MD 

Abstracting is permitted with credit to the source. 
Libraries are permitted to photocopy articles in this 
volume beyond the limits of U.S. copyright law if for 
private, noncommercial use of patrons. For other copy- 
ing, reprint or republication permission, write to Elec- 

tronic Conventions, Inc., 999 North Sepulveda 
Boulevard, El Segundo, California 90245. All rights 
reserved. Southcon 1982 Professional Program Papers. 
Copyright© Electronic Conventions, Inc. 


Alan A. Schneider, Ph.D. 
Catalyst Research Corporation 
1421 Clarkview Road, Baltimore, Maryland 



As more computing power is being 
built into devices such as terminals, 
cash registers, data loggers, etc., 
more attention must be directed to how 
these devices will function during and 
after main power interruption. A de- 
sirable, and almost necessary feature 
is that such devices power up in a con- 
dition that allows them to continue 
their function without reinitialization 
by the operator. This means that de- 
vices like system time/date clocks 
should continue to run and that vola- 
tile data should be retained. 


There are many methods by which 
data retention might be accomplished. 
Core memories have been used, but they 
are large, require more power and are 
expensive. The same can be said for 
other conventional storage media such 
as hard disk and floppy disk. Bubble 
memories are an alternative but infor- 
mation retrieval is slow because access 
is sequential and expense is also a 


EAROM's and E PROM's are a popular 
alternative but even these have their 
drawbacks — access times are measured 
in tens of milliseconds, the number of 
reliable writes is finite, and there is 
a sizeable software burden associated 
with storage of data after detection of 
incipient power failure and another to 
restore function after power up. 

Shadow RAM's offer a method of 
avoiding some of the software burden by 
transferring with a single command the 
contents of a RAM to an E 2 PROM contain- 
ed in the same package. The burden of 
detecting incipient power failure still 
rests with the user. 


Perhaps the most straightforward 
method of assuring data retention is to 
supply continuous power to the CMOS RAM 

chips which are already in use on the 
board. Continuous battery power is 
also the only way by which devices 
like clocks can continue to function 
during power down. 

Several companies provide batter- 
ies for RAM backup applications and 
at least one provides a battery-RAM- 
switch module which senses impending 
power failure and provides a signal 
to the processor that the RAM will soon 
be on battery backup in a write-pro- 
tected mode. 


Not all batteries are suited for 
CMOS RAM backup. Some are short lived, 
some leak, some have a questionable 
safety record. The ideal battery for 
CMOS RAM backup should exhibit these 
characteristics : 

(1) Printed circiut board mountable 

(2) Wave solderable 

(3) Provide data retention for the 
life of the product 

(4) Retain data over a wide temper- 
ature range 

(5) Not require recharging 

(6) Not require replacement 

(7) Be hermetically sealed to 
prevent leakage 

(8) Have a proven record of 

(9) Exhibit a high degree of safety 

To this point, two generic battery 
types have been used as power backup 
systems: Nickel-cadmium rechargeables 
and several lithium systems. Recharge- 
ables have several disadvantages: Some- 
what narrow operating temperature range; 
replacement required after about four 
years of service; recharging electronics 
is required; leakage problems can occur; 
and backup time is limited to a few 
months in the absence of recharge. 
They are an excellent choice, however, 
when high currents or low cost are im- 
portant factors. 




Lithium batteries in general can 
provide very long service life, ability 
to operate over a wide temperature 
range, and high energy' density (watt- 
hours/cm^), factors which are essential 
for backup and which conventional bat- 
teries do not provide. Almost all are 
hermetically sealed and some are PC 
board mountable using wave soldering 
techniques. However, lithium batteries 
bring with them their own set of con- 
cerns for designers: 

(1) Some are limited in the maxi- 
mum current they can provide 

(2) Safety is a concern with some 

(3) Some systems have not been 
available for more than a few 
years so long-term reliability 
may be questionable. 

"Lithium battery " is a generic 
term. It is important that the de- 
signer recognize the differences among 
the lithium battery systems and choose 
the one best suited for his application. 
It is not wise, for example, to choose 
a lithium battery only on the basis 
that it has the highest capacity rat- 
ing and fits in a particular slot on 
a PC board. That same cell, in a few 
years, may leak or lose much of its 
capacity to internal discharge pro- 
cesses . 


There are three basic types of 
lithium cells which have been offered 
for memory backup applications: 

(1) Lithium-inorganic electrolyte 

(2) Lithium-organic electrolyte 

(3) Lithium-solid electrolyte 

All provide a distinct advantage over 
conventional cells in terms of watt- 
hours per unit volume and all have the 
promise of long life. 

Lithium-Inorganic Electrolyte Batteries 

Only one version of this type cell 
has been seriously promoted for use in 
backup applications — the lithium- 
thionyl chloride (Li-SOCl-) cell. It 
is manufactured by Tadiran, GTE, Altus 
and others. 

In addition to the lithium anode, 
the cell uses thionyl chloride (S0C1 2 ) 
the electrolyte solvent and a high-sur- 
face-area carbon as the cathode current 
collector. SOCI2 serves not only as 
the electrolyte solvent but also as the 
active cathode species. Because one 
component serves two roles, the cell is 
very energetic (Ca. 1.1 watt-hours /cm3 ) . 
It functions well over a wide temper- 
ature range and has an attractive 
single cell voltage (>3.5 volts). It 
is capable of delivering moderate to 
high current (1-1000 ma) depending on 
cell design. 

The system has also shown several 
disadvantages over the past few years. 
Self-discharge, the rate at which the 
cell consumes its own capacity intern- 
ally, can be very high and unpredict- 
able. Several manufacturers claim to 
have solved this problem, but multiyear 
testing is necessary to be certain. 

Perhaps the biggest question with 
this system is safety. Very large bat- 
teries, designed for the military, have 
shown a few serious problems. Certain- 
ly the smaller versions of this cell 
with limited current capability are 
much safer. Moreover, several manu- 
facturers claim to have solved the 
safety problems by mechanical means or 
by the use of additives. Nevertheless, 
some questions about this system still 
exist in the minds of battery profess- 
ionals, and the electronics system de- 
signer would do well to be certain that 
for his application, all the safety re- 
quirements are met. Again, time and 
the testing of very large numbers of 
cells will tell the story. 

Lithium-Organic Electrolyte Batteries 

At least three types of lithium- 
organic electrolyte cells have been 
offered for memory backup: 

(1) Lithium-manganese dioxide 
(Li-Mn0 2 ) 

(2) Lithium-carbon fluoride 
(Li-CF X ) 

(3) Lithium-sulfur dioxide 
(Li-S0 2 ) 

The Li-Mn0 2 system is offered by 
Sanyo, Ray-O-Vac , Mallory and others. 
It is a moderate rate system (0.1-10ma) 



with a voltage near 2.8 volts. In addi- 
tion to the lithium anode, the cell uses 
a pellet containing MnC>2 as the cathode 
and an electrolyte composed of an in- 
organic salt dissolved in an organic 
solvent. Its energy density is moder- 
ate (Ca. 0.6 watt-hours /cm3 ) . It can 
operate over a fairly wide temperature 
range with low self-discharge. It has 
thus far shown itself to be a safe 
system and has promise to be a long- 
life (5+ years) system. 

Its principal disadvantages are 
lower cell voltage, lower energy den- 
sity, and lower rate than Li-SOCl2- 
To many designers, these are outweighed 
by its advantages. 

The Li-CF X system is offered by 
Panasonic, Eagle-Picher and others. It 
is similar in most aspects to the 
Li-Mn02 systems except for a slightly 
higher current capability. 

The Li-SC>2 system is different 
from the other lithium-organic electro- 
lyte in that it uses a soluble gas 
rather than a solid pellet as the cath- 
ode. It can build up pressure as it is 
discharged or heated and some of the 
larger cells use safety vents to pre- 
vent case rupture. A few safety-re- 
lated incidents have been recorded with 
some of the larger cells, but certainly 
the smaller cells should cause little 
or no problem. Its energy density is 
about the same as other Li-organic 
systems and it operates over a rather 
wide temperature range. Its current 
capability is somewhat higher than 
Li-MnC>2 or Li-CF X . 

Lithium-Solid Electrolyte Batteries 

Lithium-solid electrolyte systems 
are provided by Catalyst Research, 
Mallory and others. Unlike other lith- 
ium batteries, the electrolyte is solid, 
a distinct advantage for long life. 

The solid electrolyte system which 
is most used is the lithium-iodine 
system. Its electrolyte is solid lith- 
ium iodide which accumulates as the 
cell is discharged. Unlike other cells, 
this cell uses no separator to prevent 
internal shorting of anode and cathode. 
The electrolyte itself is the separator, 
and should it form a crack, it "heals" 

itself. This is another important 
factor in the performance of this long- 
life system. The cathode is a pellet 
of iodine mixed with a small amount of 
organic to make it conductive. 

This system is the cell of choice 
of the pacemaker industry, powering 
more than 90% of the worlds pacemakers. 
It boasts a real 10-year data base en- 
compassing more than a million cells. 

Its energy density is quite high 
(Ca. 0.9 watt-hours /cm^) and its abil- 
ity to deliver current is somewhat low 
(0.1 - 2 ma), but still high enough for 
data retention for many CMOS RAMS. 
Open circuit voltage is 2.8 volts. 

There have been no safety-related 
incidents in its 10-year history. 


Once the choice of battery has 
been made, the interfacing to the pro- 
cessor or other electronics may seem 
trivial. In fact, this is far from 
true. The designer must worry about 
the characteristics of the switch, 
logic level incompatibilities created 
by two power supplies, and drawing too 
much current from the battery during 
switch transitions. These and other 
topics will be covered in a later 
paper . 


Battery backup seems to be the 
ideal solution for data retention and 
powering devices like real-time clocks. 
When used with CMOS RAM's, battery back 
up offers the speed and cost advantages 
of RAM storage with the additional ad- 
vantages of non-volatility. 

Lithium batteries offer real ad- 
vantages over conventional and re- 
chargeable batteries, but care "must be 
exercised to find the right lithium 
battery. Factors such as safety, en- 
ergy density, and proven reliability 
must be considered. 

With the proper choice of battery 
and interfacing electronics, it is pos- 
sible today to design a system with a 
backup battery that can last the life 
of the product, a system in which the 
battery is treated like any other non- 
replaceable component and soldered 
directly to the PC board. 




Thomas Irwin 
Application Engineer 
RAYOVAC Corporation 
101 East Washington Avenue 
Madison, WI 53703 


Since the development of the elec- 
trochemical storage battery almost a 
century ago, a growing number of appli- 
cations have employed batteries both as 
their only power source and as a back 
up supply during commercial power out- 
ages. In the last 100 years, a variety 
of battery chemistries have evolved both 
in primary and secondary batteries. 

Today we are entering a microcom- 
puter age where the demand for standby 
power to preserve volatile memories and 
to insure uninterrupted clock operation 
is increasing rapidly. Applications for 
standby power are in computers or process 
control equipment using real time clocks, 
computer printers where the customer 
initializes his printing format in 
volatile memories, portable data ter- 
minals used in inventory control, tele- 
communications, portable test equipment, 
and a growing list of other applications. 

Miniature batteries are becoming an 
important tool to the microcomputer 
system designer. When complimented with 
power line sensing and write inhibit 
semiconductor devices, it enables the 
designer to use standard NMOS and CMOS 
RAMs without risking program or data 
loss. The designer can then continue 
using high speed low cost memories with- 
out sacrificing write speeds or having 
to resort to EPROMs or high priced low 
speed multi-voltage EEPROMs. Nor, is 
the number of write operations limited 
to some number X due to wear-out 
phenomena . 

The purpose of the battery is to 
protect the microcomputer, its volatile 
memory, its clock circuitry, or any com- 
bination of these from AC line distur- 
bances. To design a battery backup 
system, an engineer needs an under- 
standing of power line disturbances. 
The types of abnormal power line con- 
ditions to be reckoned with are 1) 
Transients: a short term voltage or 
current disturbance having a duration 
between 1 nanosecond and 5 milliseconds. 
2) Voltage Sags: a drop in voltage 

lasting more than 5 milliseconds but 
less than 5 seconds. The voltage value 
must be below the lower specification 
limit for nominal voltage. 3) Power 
Interruption: a complete loss of voltage 
for any period greater than 5 milli- 
seconds, but less than 500 milliseconds. 
4) Power Outage: a complete loss of 
voltage for any period exceeding 500 
milliseconds. 5) Brownout: a deliberate 
reduction in nominal voltage supplied by 
the utility company. The value may fall 
at or below the lower limit specified 
for nominal voltage. 

To quantify the problem, IBM did a 
study-'- of AC power sources for typical 
data processing equipment. The study 
involved 49 locations and 125,000 hours 
of monitoring. The study was made from 
July 1969 to July 1972. The information 
is broken down into duration of distur- 
bance and percent of voltage lost. 

During the 125,000 hours of mon- 
itoring 1,790 voltage sages, power 
interruptions, and power outages 
occurred; that is, one disturbance every 
69 hours of operation. 

Assuming the power supply can op- 
erate down to 80% of nominal line 
voltage, Figure 1 shows line disturbances 
that will cause memory problems. In 
125,000 hours of operation there would 
be 1,040 disturbances, or one problem 
every 120 hours of operation. With the 
addition of a battery backup system, 
errors due to line disturbance can be 
controlled . 

Additional considerations to be 
aware of are the electronic cash 
registers which may be shut off at night 
or on weekends and without battery backup 
could encounter loss of sales totals 
and/or inventory sold. Computerized 
office equipment involved in office 
moves and rearrangements can be subjected 
to power outages of several minutes, 
hours, or days. Without battery backup 
initialized data, accumulated data, and 
clock functions can be lost. 














.5 to 







































































































One additional problem is worth 
mentioning. Some computer products are 
manufactured and shipped with pre- 
programmed information. Batteries must 
be used for memory retention in the 
interim between shipment and instal- 
lation by the end user. 

The purpose of this paper is two- 
fold: 1) to review the history of 
primary and secondary batteries used for 
memory backup applications; and 2) to 
present the most recent developments in 
lithium batteries as they relate to the 
environment . 

Before I get into the different 
battery systems, I'd like to establish 
some general characteristics of an 
"Ideal Power Cell" and use it as the 
yardstick to which all batteries are 
compared and measured. The "Ideal Power 
Cell" would have: 

1. Maximum Energy Density - offer a 
maximum amount of energy stored in 
a minimum amount of space. 

2. Unlimited Shelf Life - no self- 
discharge irregardless of storage 

3. Rechargeable - fully rechargeable at 
any charging rate. 

4. Light Weight - combine maximum energy 
density with a minimum amount of 
weight in the materials used. 

5. Safe - the ideal cell would be com- 
pletely safe - no leakage or gassing. 
The contents would be harmless to 
people and the environment. 

6. Good Performance - deliver the 
required current and voltage under 
a wide range of temperature and 
load conditions. 

7. Cost - the materials used to make 

the cell would be in plentiful supply, 
multisourced , and manufacturing costs 
would be minimal. 


Nickel-Cadmium Batteries 

Nickel-cadmium rechargeable bat- 
teries are secondary batteries. They 
were first developed in the 1940 's. 
Figure 2 is an overview of several bat- 
tery systems and their respective energy 
densities. Although the nickel-cadmium 
battery has a low energy density, the 
fact that it can be recharged compensates 
for this characteristic 

Advantages of nickel-cadmium bat- 
teries are: they offer excellent rate 
capabilities, have a stable discharge 
voltage, perform very well at low tem- 
peratures (-20°C and below) , and have an 
operational life of 3 to 5 years, or 500 
to 1000 charges, whichever comes first. 

Disadvantages are: it has a low 
energy density, an individual cell 
voltage of 1.2 volts requiring a stack 
of several cells to form a usable battery 
for memory backup/clock applications, a 
high self-discharge rate of 1% per day 
at room temperature and several percent 
per day at temperatures above 4 0°C, and 
if subjected to many shallow discharge/ 
charge cycles can develop a "memory 
effect" where it can fail to deliver its 
full capacity when called upon to do so. 

Figure 3 compares the nickel-cadmium 
to the ideal power cell. 

Most applications employ batteries 
consisting of a 3 or more cell stack. 
Typical applications are for memory 
backup/clocks, calculators (LED), test 
equipment, toys, etc. 

Sealed Lead Acid 

Sealed lead acid rechargeable bat- 
teries are members of the secondary bat- 
tery family. Conventional lead acid 
batteries with the liquid electrolyte 
have been available for over seventy 
years. Sealed lead acid batteries have 
been available for about 7 years. 
Figure 2 compares its energy density to 



other battery systems. Although sealed 
lead acid batteries have a low energy 
density, the fact that they can be re- 
charged compensates for this 
characteristic . 

















S0Cl 2 /Li 












CF x /Li 




























CELL SIZE: 11.6 x 1.2 mm 


Advantages of sealed lead acid 
batteries are: they have a nominal cell 
voltage of 2 volts, can deliver high 
currents, have a flat discharge, can 
operate over a temperature range of 
-40°C to +60°C, operational life of 3 to 
5 years or 100 to 250 charges, and an 
ability to tolerate cell reversal with- 
out damage. 

Disadvantages are: its low energy 
density, a self-discharge rate of 6% 
to 8% per month at room temperature, 
limited number of recharges, the smallest 
size available is comparable to a "D" 
size, and it is physically heavy. 

Figure 3 compares the Sealed Lead 
Acid battery to the ideal power cell. 

Sealed Lead Acid batteries are 
available in sizes from 2.5 AH to over 
25 AH and in 2 volt increments from 2 
volts to 48 volts. Typical uses are 
standby power for powering large blocks 
of memory at a 2 volt data retention 
level, uninterruptable power supplies 
for small to medium size computers, 
alarms, emergency lighting, and as 

cyclic power for tools, instruments, 
engine starting, televisions, and 
video tape recorders. 

Mercury Batteries 

Mercury primary batteries were 
first mass produced during World War II. 
Figure 2 shows mercury's energy density 
relative to other battery systems. As 
you can see, mercury is very high in the 
table and at the time of development was 
the highest energy density battery 
available. This characteristic also 
facilitated development of miniature 
button cell batteries having capacities 
in the hundreds of milliampere-hours . 

Advantages of mercury batteries are 
their high energy density, flat dis- 
charge characteristic, good rate capa- 
bility at currents from microamperes to 
hundreds of milliamperes , and an oper- 
ating temperature range of -2 0°C to 

Disadvantages of the battery are: 
a cell voltage of 1.35 volts, a shelf 
life at room temperature of 1-2 years, 
problems of mercury migration during 
discharge which can internally short out 
the cell, the environmental concern of 
mercury disposal, and relatively high 

In some of the first computerized 
cash registers, 1 AH mercury batteries 
were used for memory backup. However, 
it was found that the batteries did not 
stand up well under the daily temper- 
ature cycling. Too often the cells 
would leak or go dead after one-half of 
their capacity had been expended. 
Coupled with the 1-2 year shelf life, 
the mercury battery bowed out of the 
memory backup market. 

Figure 3 compares the mercury bat- 
tery to the ideal power cell. 

Present day applications for the 
mercury battery are in hearing aids, 
cameras, instruments, and smoke detec- 
tors, which benefit from its high energy 
density characteristics. 

Alkaline Batteries 

The alkaline manganese, or simply 
alkaline battery as it is commonly 
called, was developed about 25 years 
ago. While most of the alkaline bat- 
teries available today are primary 







SHELF LIFE Excellent 







Low/Med Low 



















♦Depends upon particular system 


batteries, there is a rechargeable 
version. Figure 2 is an overview of 
several energy density chemistries. As 
you can see, Alkaline Manganese is an 
improvement over the zinc carbon and 
nickel-cadmium, but has only 40% of the 
energy density of mercury. 

Alkaline batteries are available 
in sizes from 40 MAH button cells to 
multiampere hours "D" cells and battery 
voltages from lh volts to 9 volts. 

Advantages of the alkaline battery 
are its high rate efficiency, improved 
shelf life of 3-5 years, good to excel- 
lent low temperature performance, cell 
voltage of 1.55 to 1.6 volts, and low 

Disadvantages are its low to 
moderate energy density. Alkaline 
button cell batteries have insufficient 
energy to meet memory backup power re- 
quirements for several years of oper- 
ation. Elevated temperatures accel- 
erate the self-discharge rate, limiting 
its overall service life. 

Figure 3 compares the alkaline 
manganese battery to the ideal power 

Present day applications for this 
battery are LCD calculators, cameras 
(exposure control system and strobes) , 
and toys. 

Silver Oxide 

Silver oxide batteries were 
commercially developed about 10-15 years 
ago. There are two similar chemistries 
used in commerical silver oxide button 
cell batteries, monovalent and divalent 
silver oxide. 

Figure 2 lists the energy density 
of both silver oxide chemistries. Mono- 
valent silver oxide has the formula 
Ag 2 and falls between alkaline and 
mercury. Divalent has the formula AgO 
and offers the highest energy density 
outside of the Zinc-Air system. (The 
mechanics of activating/deactivating a 
Zinc-Air battery prevents its use in 
battery backup applications.) The high 
energy densities lend themselves to 
miniature button cells. Almost all 
commercially available silver oxide 
batteries are button cells. The devel- 
opment of larger cylindrical size silver 
oxide batteries is possible but is 
restricted by the price of silver. 

Characteristics of the monovalent 
and divalent cells are their high energy 
densities - about 2-2^ times that of 
alkaline, an operating voltage of 1.55 
volts, light weight, good low and high 
rate current capacities, a shelf life 
at room temperature of 3 to 5 years in 
high rate cells, and 4 to 6 years in 
low rate cells, good low temperature 
operation, and low leakage rates. 


Disadvantages of both types are the 
increased self-discharge rates which 
occur when cells are subjected to high 
temperatures for extended periods of 
time. The fluctuating market price for 
silver in the past couple of years has 
increased silver oxide button cell 
prices significantly. Currently, silver 
is below $10/Troy Oz . and it is again a 
viable cost product. 

Differences between mono and 
divalent cells occur in two areas. 
First, divalent has a higher energy 
density, about 30% more, than mono- 
valent. In the same size cell, divalent 
can offer significantly more energy. 
Second, divalent chemistry produces a 
higher cell voltage of 1.8 volts com^ 
pared to 1.55 volts for monovalent. In 
commercial divalent silver oxide cells, 
special techniques are used to reduce 
the voltage to 1.55 volts without 
sacrificing the higher energy density. 

In terms of low temperature oper- 
ation, a divalent silver oxide watch 
cell measuring 11.56 mm diameter by 5.36 
mm high and rated at 1.5V, 220 MAH, has 
delivered 113 microamperes at -70°F at 
a closed circuit voltage of 1.44 volts. 

Silver oxide button cell batteries 
range in size from 2.10 mm high by 6.80 
mm diameter to 5.36 mm high by 11.56 mm 
diameter and capacities of 17 MAH to 
220 MAH. 

Figure 3 compares both silver oxide 
battery systems to the ideal power cell. 

Lithium Batteries 

The most exciting new battery 
system entering the marketplace today 
is lithium. Actually, it should be 
referred to as lithium systems since 
there are in excess of 10 different 
systems that use lithium. 

Figure 4 is a partial listing of 
companies and the lithium technologies 
they are or have investigated. 

The primary incentive for the 
development of lithium batteries is 
their potential for very high energy 
densities which in small batteries can 
realistically be expected to be 450 to 
760 Watt-Hours/Liter delivered. Another 
incentive is the higher cell voltage of 
2.6 to 3.6 volts. Lithium batteries 
also exhibit shelf lives of 5 to 20 



RAYOVAC CORP. Mn02, CuS, Cu 2 S, S0Cl 2 , Ac^CrO^ FeS, CF x , 


SANYO Mn0 2 


UNION CARBIDE Mn0 2 , FeS 2 , S0Cl 2 

MALLORY Mn0 2 , SOj 



HONEYWELL S0Cl_ 2 , V 2 5 

GTE S0Cl 2 


VARTA Mn02, Bi 2 3 






SAFT CuO, Bi 2 Pb 2 5 , Mn0 2 > S0Cl 2 , AG 2 CR0t| 

PCI S0 2 



years depending upon the particular 
lithium system. They can also operate 
at low temperatures to which conventional 
•batteries would be inoperative. 

Lithium is the lightest metallic 
element known to man and is very re- 
active. It is a far more reactive 
material than zinc and even surpasses 
potassium and sodium. The extreme 
reactivity of lithium metal, which 
theoretically makes it such an attrac- 
tive anode material, also makes it very 
difficult to work with. In the presence 
of water vapor or water it can ignite 
spontaneously. Hence, assembly must 
take place in dry rooms and the seals on 
the cells must be of a very high quality. 

In terms of lithium chemistries 
which are at a production or prototype 
level, the major ones are solid state, 
sulfur dioxide (S0 2 ) , carbon monofluoride 
(CF X ) , manganese dioxide (MnC^), and 
thionyl chloride (SOCI2). Figure 2 
compares the energy densities for these 
lithium systems. Figure 5 is a graph 
of their discharge characteristics. 




Lithium/Manganese Dioxide 

CELL SIZE: 11.6mm O.D. x 1.2mm HT. 
DRAIN: 5 Hicroamp 

10 SO 

120 160 200 210 
CAPACITY (mAh/cell) 
Characteristic Discharge Voltage of Energy Systems 

Solid State 

Lithium solid state batteries are 
as their name implies, composed of a 
solid anode, solid electrolyte, and 
solid cathode. Combined with hermetic 
sealing, they may achieve shelf lives of 
10 to 20 years or longer. 

Solid state batteries were ini- 
tially used in cardiac pacemakers where 
high reliability, low drains, and long 
operational life were extremely impor- 
tant. As power requirements for CMOS 
memories were reduced to the low micro- 
ampere range, a new application for 
solid state batteries developed. 

Solid state batteries offer a high 
energy density, approximately twice that 
of the alkaline battery, a long shelf 
life of 10 to 20 years, a wide temper- 
ature operating range of -55°C to +125°C, 
no leakage, a cell voltage of 2.8 volts, 
and a flat discharge characteristic. 

In button cell sizes the principal 
limitation is a typical current drain of 
20-25 microamperes, 50 microamperes 
maximum. They also have a moderate to 
high cost. 

As the standby power requirements 
for CMOS RAMs continue to fall into the 
low and sub-microampere range, more 
applications will develop for solid 
state lithium batteries. 

Lithium manganese dioxide cells 
consist of a lithium anode, separator, 
manganese dioxide cathode, and an 
organic electrolyte. They have a 
nominal cell voltage of 3 volts and 
deliver approximately 2.9 volts under 
load. Its energy density is better than 
twice that of an alkaline battery. They 
usually employ crimp seals but can use 
a hermetic seal if the application so 
requires. The shelf life is estimated 
at 5 to 10 years. Lithium manganese 
dioxide cells are available in button 
cell and cylindrical sizes ranging in 
capacity from 30 MAH to 1000 MAH. 

Advantages are its high energy 
density, 3 volt cell voltage, good rate 
capability, low self-discharge rate, has 
a temperature operating range of -2 0°C 
to +50°C, and a sloping discharge which 
lends itself to end of battery life 

Lithium/Carbon Monofluoride 

Lithium carbon monofluoride cells 
consist of a lithium anode, separator, 
carbon monofluoride cathode, and an 
organic electrolyte. The nominal cell 
voltage is 3 volts and 2.8 volts under 
load. It has an energy density slightly 
higher than that of lithium manganese 
dioxide. Lithium carbon monofluoride 
cells are available in sizes from button 
cells to "C" size cylindrical cells. 
Crimp seals are used in both the button 
and cylindrical cells. Shelf life is 
estimated at 5 to 10 years. Capacities 
range from 40 MAH to 5 AH. The button 
cells can deliver continuous currents 
up to 250 uA with pulsing up to 10 milli- 
amperes. The larger cylindrical cells 
can deliver currents from microamperes 
to hundreds of milliamperes . 

Advantages are its high energy 
density, cell voltage of 3 volts, good 
rate capability, low self-discharge 
rate, and an operating temperature 
range of -20° to +60°C. 

Lithium Thionyl Chloride 

Lithium thionyl chloride cells are 
a high voltage, high energy density 
lithium system. Figure 2 shows lithium 
thionyl chloride at the top of the list. 



If lithium systems were classified 
as low, medium, or high rate, thionyl 
chloride would fall in the high rate 
category. It has a nominal voltage of 
3.6 volts and is commercially available 
in sizes from .85 AH to 10.8 AH. 

They are available both in low 
profile prismatic and conventional 
cylindrical shapes. The basic con- 
struction consists of a lithium anode, a 
separator, a carbon cathode, and a 
thionyl chloride electrolyte/depolarizer. 

Advantages are: a very high energy 
density, a high cell voltage, excellent 
rate capability, with hermetic seals - a 
shelf life of up to 10 years, a very low 
self-discharge rate, and a temperature 
operating range of -55°C to +70°C. 

Disadvantages of the system are its 
susceptibility to a voltage delay problem. 
If initially unused for some period of 
time, a passive layer develops on the 
anode. If the cell is subjected to a 
light current drain, the voltage drop 
will be very small to non-existant . It 
is only on a heavy drain that the cell 
voltage drop is pronounced. The delay 
time during heavy drains can be several 
seconds in length. Under both light and 
heavy drains the layer is dissipated. 

Lithium Sulfur Dioxide 

Lithium sulfur dioxide cells have 
been in production for several years, 
primarily for military markets. They 
are available in sizes of .43 AH up to 
30 AH. The nominal cell voltage is 3 
volts and 2.8 volts under load. The 
basic cell is spiral wound consisting of 
a lithium anode, a separator, a carbon 
cathode, and a sulfur dioxide rich 
electrolyte. The cells are usually 
hermetically sealed. The sulfur dioxide 
cells are operated at a positive internal 
pressure which can reach 100-200 PSI 
(pounds per square inch) at elevated 
temperatures. The operating temperature 
range is -65°F to +165°F. At temper- 
atures between 2 30°F to 25 0°F and 
pressures of 450 to 500 PSI, a safety 
vent is activated rendering the cell 
inoperative. The sulfur dioxide system 
can sustain currents from microamperes 
to amperes in its "D" cell size. Shelf 
life is estimated to be about 10 years. 

Lithium sulfur dioxide cells are 
also susceptible to the same type 
voltage delay seen with lithium thionyl 
chloride. Here too, good progress is 
being made to eliminate the problem. 

17/1 7 

Lithium as a composite battery 
system has the advantage of a high 
energy density, a cell voltage of 2.6 
to 3.6 volts, a wide temperature oper- 
ating range, greatly improved shelf 
life, low leakage rates, light weight, 
good rate capabilities and long term 

Lithium is not a high current drain 
battery system. On an equivalent size 
basis to an alkaline battery, it cannot 
supply the same high currents to a load. 
Lithium is high priced primarily because 
it is not yet in mass production. There 
is a lack of universal availability and 
there are government restrictions on 
shipment of lithium cells having over 
one-half gram of lithium. 

Today's applications for lithium 
button cells are strongest in the watch 
and calculator markets. However, a 
growing number of applications for > 
button cells as small as the 4 MAH to 
as large as multiampere hour thionyl 
chloride cells, are appearing in indus- 
trial controls, test equipment, telephones , 
memory backup, and computer clock/timing 
functions . 

Figure 3 compares the lithium systems 
with the ideal power cell. 


The microelectronic system designer 
now has a broad portfolio of battery 
systems with which to meet his backup 
power requirements. Secondary batteries 
like the Nickel-Cadmium and Sealed Lead 
Acid should be considered where the 
application could require frequent re- 
placement of primary batteries, where 
high current drains are required, where 
primary batteries may not be replaced, 
and if rechargeability makes the device 
more desirable to the user. Primary 
batteries like the silver oxide and 
lithium systems can offer many years of 
reliable protection for volatile 
memories and clocks, can reduce mainte- 
nance, and in some cases may be good 
for the life of the equipment, can 
eliminate the need of charging circuitry, 
can operate over broad temperature ranges , 
and can be directly mounted on the 
printed circuit board. 

While this paper presents some 
design guidelines for selecting and 
applying primary and secondary batteries, 
it is by no means complete. It is in- 
tended as a guide to you, the systems 
designer, for determining your battery 
requirements . 

In the process of making your bat- 
tery selection, it is very important to 
solicit more detailed information and 
assistance from the battery manufac- 
turers. Generally, manufacturers have a 
group of trained application engineers 
to provide technical assistance and 
specification guidelines. By working 
with the battery manufacturer, you can 
minimize risk in product design, 
optimize product performance and con- 
tinue the successful alliance of bat- 
teries with microelectronics. 


1. Memory Support for Data Processing 
Equipment; Mr. David Kuykendall, 
1979 Wescon Professional Program, 
Session #12. 

2. Power Sources for Volatile Memories 
Joseph Carone, 1980 Wescon Pro- 
fessional Program, Session #24. 




Dr. Isamu Kuru, 
Msayoshi Nakane, 

Kaoru Tokushige 
Toshiba America Inc. 
2151 Michelson Drive 
Irvine, California 92715 

It was becoming widely acknowl- 
edged that the Complimentary-Metal 
Oxide-Silicon (C-MOS) process provides 
many advantages over other processes in 
fabricating modern integrated circuits 
such as Random-Access-Memories (RAMS) 
and logic circuits. The major 
advantages of the C-MOS process over 
other MOS processes are 1) wider 
operating temperature range; 2) wider 
operating voltage range; 3) higher 
noise margins; and 4) low power 
consumption. Until recently, the 
major disadvantage to the C-MOS process 
has been slow operating speed but, as 
we shall show in this paper, this 
disadvantage has been overcome by 
advancement in both circuit design and 
process capability by Toshiba 
Corporation which has culminated in the 
very popular TC5516/17/18 series of 16K 
static C-MOS RAMs currently in 

The ability to produce a high- 
density RAM with fast access time (200 
nSec) and exceptionally low power 
consumption (2-3 nA) during stand-by 
operations has opened many new 
application opportunities where 
non-volatile memory retention or low 
operating power are key issues. This 
article will describe this new family 
of ultra low standby power CMOS RAMs 
and their use in non-volatile battery 
backup applications. 


The characteristics which make 
CMOS RAMs ideal devices for non- 
volatile memory storage applications 
are their very low power dissipation 
and their very low voltage requirement 
(2V) during standby operation where 
the only function of the device is to 
retain the memory information already 
stored in the device. 

Figure 1 through 3 are presented 
to describe how these merits are 
achieved in CMOS RAMs. 


Figure 1 describes a typical CMOS 
memory cell structure. This basic 
circuit is used to store one bit of 
digital information and is replicated 
many times within the completed RAM 
device. This circuit is known as a 
"bi-stable flip-flop". The bi-stable 
flip-flop has only 2 stable operating 
modes: either Trl and Tr4 are both on 
(resulting in the storage of a "0" 
logic level) , or Tr2 and Tr3 are both 
on (resulting in the storage of a "1" 
logic level) in the cell. Either of 
these two stable conditions will be 
maintained as long as VDD is larger 
than the sum of the voltages necessary 
to turn on one P-channel transistor 
(Trl or Tr2) and one N-channel tran- 
sistor (Tr3 or Tr4) . These two 
voltages are essentially the threshold 
voltages of the transistors and 
usually are in the range of 1-2.0 
volts and it is this characteristic 
that gives CMOS RAM the ability to 
maintain stored data with VDD down to 
the 2V level. 

Further investigation of this 
circuit indicates that when the bi- 
stable flip-flop is in either of its 
two stable states, no DC path exists 
between VDD and VSS because one pair 
of transistors is always off. This 
characteristic results in the very 
low standby current consumption of 
the CMOS RAM during inactive periods 
of operation. 

Figures 2 and 3 describe this 
structure in more detail. Figure 2 
shows a cross section of the CMOS 
inverters used to implement a memory 
cell. Figure 3 is a schematic 
representation of the inverter. When 
this inverter circuit is in either of 
the two stable conditions, it can be 
seen that the P-channel transistor 
and the N-channel transistor cannot be 
turned on simultaneously. This means 
that essentially no power is 
dissipated in the inverter when the 
circuit is in a stable state. 

In reality, however, a very small 
amount of power is dissipated even in 
the stable states because the tran- 
sistors are not "ideal" and therefore 
a small leakage occurs even in the off 
condition. The magnitude of the 
leakage current is proportional to the 
area of the p-n junction used to form 
the transistor, and, in the case of 
the TC5516/17/18, the dominant 
component arises from the junction 
formed by the Pwell and the N substrate 
(the Pwell-Nsub junction) . In a normal 
CMOS RAM, the total area of the Pwell- 
Nsub junction is very close to the 
total chip area. Using 5mm X 5mm as 
an approximate chip size and 0.1 NA/mm2 
as a typical leakage current density at 
room temperature, the expected CMOS 
RAM leakage current in standby mode 
is 2.5 nA. 

The leakage current is dominated 
by the "Carrier Generation/Recombi- 
nation" effect and therefore is 
temperature sensitive. The expected 
change in G-R related leakage with 
temperature predicts that this leakage 
current will double with every 10 C 
increase in temperature. Thus, if InA 
of leakage is observed at 25 C, the 
expected leakage of the same device at 
85 C would be 64nA. 

Low Standby Power CMOS RAM 

As was described above, the 
standby power dissipated in a CMOS RAM 
is determined by the leakage current 
across the reverse-biased P-N junction 
in the device. In the actual device, 
the dominant causes of Generation- 
Recombination Centers in the device 
are metallica impurities found in the 
depletion layer of the P-N junction. 
These metallic impurities in the 
depletion layer act as additional 
Generation-Recombination Centers 
(beyond the normal expected due to 
crystal imperfections or intersticial 
sites) and therefore add significantly 
to the background leakage current. 

Toshiba has developed proprietary 
CMOS process technology which enables 
the reduction of these metallic 
impurities in the P-N junctions by a 
significant amount. Toshiba utilizes 
this process in the production of the 
currently available TC5516 APL device, 
a 16K Static CMOS RAM. Typical 
standby current values at room 
temperature for this device are 2-3 nA. 
Figures 4 and 5 describe the leakage 

current characteristics of the TC5516 
APL versus voltage and temperature. 
Figure 6 is the standby current dis- 
tribution data for a typical diffusion 
lot. Approximately 9 0% of the 
distribution have standby current less 
than 10 nA and the center of the 
distribution is 2-3 nA. 

The above standby characteristics 
make the TC5516/17/18 APL devices very 
attractive and practical for non- 
volatile memory storage applications 
where a primary battery such as a 
Lithium Cell (rather than a 
rechargeable cell like Ni-Cd) is the 
main power source. 

Table 1 lists the current products 
from Toshiba which exhibit these very 
low standby power characteristics. 

Battery Back-up Applications 

Since these devices exhibit such 
exceptional standby characteristics, 
their major applications are in 
battery backup systems. Toshiba has 
incorporated several "easy-to-use" 
features in the fundamental design of 
the parts with these applications in 
mind . 

The first is compatiblity with 
existing N channel MOS RAMs which have 
been widely used for years. The 
RC5504 APL (4K X 1) and TC5514 APL 
(IK X 4) 4K CMOS RAMs are pin- 
compatible and functionally equivalent 
to the MK4104 (4K X 1) and i2114 
(IK X 4) NMOS RAMs respectively. The 
memory speed, which in the past has 
been one major drawback in the use of 
CMOS memory, has been dramatically 
improved in recent years and the 
timing of the TC5504 APL and TC5514 
APL are essentially equivalent to the 
corresponding NMOS RAM. This, then, 
makes it very convenient for users to 
change existing designs from 4K NMOS 
RAMs to non-volatile 4K CMOS RAMs 
without changing the memory board 

"Easy-to-use" features in battery 
backup applications was given full 
consideration in the design of the 
recently developed and marketed 16K 

The 16K CMOS RAMs are available 
in the industry standard 24 pin, 600 
mil dual-in-line package and with a 
pin-out compatible with the very 



popular 2716 16K EPROM. Additionally, 
to provide even greated flexibility 
for assembly on the memory board, two 
different pin-outs were developed. 

The TC5517 APL/BPL was developed 
to achieve pin-out compatibility with 
NMOS 16K RAMs such as the popular 
TMM2016P. The TC5516 APL and TC5518 
BPL were developed specifically for 
battery backup applications where one 
of the two available chip-enable 
signals can be used to control the data 
retention mode for the entire memory 
board. For these devices, CE2 is 
the signal used to place the device 
into the standby mode. This signal 
can be commonly connected to all 
devices on the board and when simply 
pulled to a high logic level, used to 
place all memories on the board in the 
standby, power-down, data retention 
mode . 

Another merit of the TC5516/18 
devices in battery backup applications 
is that the input voltage level 
required for all inputs to the memory 
during standby operation need only to 
be TTL compatible signals. This is 
due to the internal design of the 
device which gates all of these inputs 
with the CE2 signal in a NOR con- 
figuration. This fully eliminates 
the need for external pull-up 
resistors which are normally required 
for other types of CMOS RAM with only 
one chip-enable signal. 

Replacement of EAROM and EEPROM with 
Lithium Battery Backup TC5516 APL 

As was shown in Figure 6, the 
TC5516 APL has extremely low standby 
current. Figure 7 is an estimated 
data retention lifetime of a typical 
TC5516 APL memory system using a long 
life Lithium battery for backup. 
The worst case standby current in the 
TC5516 APL was assumed from Figure 6 
to be 50 nA, which is 5 times the 
average standby current in the device. 
A fully populated 64K byte system, 
which would use 3 2 of the above 
devices, exhibits a data retention 
time of greater than one year. An 
8K byte memory system, using 4 of the 
above devices, would have an expected 
data retention time of greater than 
10 years for just a single small 
button type Lithium cell. This is 
comparable to the predicted data 
retention time of both EAROM and 
EEPROM. Additionally, when you 

compare other characteristics such as 
write cycle time, you find that the 
CMOS RAM implementation is superior 
by several orders of magnitude. 

A third alternative over EEPROM 
and EAROM is that both of these devices 
experience a "wear-out" phenomenon 
each time information is written into 
the memory. The current predictions 
show a maximum number of writer cycles 
for each of these devices on the order 
of 10 5 cycles. In any common computer 
or microprocess application, 100,000 
write cycles can be accumulated very 
early in the installed life of the 
equipment making these non-volatile 
types of memories unusable for a 
majority of applications. CMOS RAM 
does not experience this wear-out 
phenomenon and therefore can be 
written to an unlimited number of 
times without wearing out. A major 
and distinct advantage of CMOS RAM 
over EAROM and EEPROM. 

Basic Circuit for Battery Backup 
CMOS Application 

Figure 8 shows the basic circuit 
used to provide backup power from a 
battery to a CMOS RAM. When the 
system power supply is at its normal 
operating voltage, diode Dl is 
conducting (passing current) and diode 
D2 is non-conducting, which isolates 
the CMOS RAM from the battery. In 
this condition, the actual voltage 
provided to the CMOS RAM is VS (power 
supply voltage) - VFl (forward voltage 
drop of diode Dl) . When the power 
supply fails and VS begins to fall, 
diode D2 will begin to conduct when 
VS-VF1 < VB-VF2 is established. The 
resulting voltage to the CMOS RAM 
will then be VB (battery voltage) - 
VF2. Since this value can drop all 
the way to 2V using the TC5516 APL, 
data retention will be guaranteed for 
battery voltages down to approximately 

This circuit is the simplest 
possible backup mode and operates in 
a fully passive mode. 

Battery Backup Memory Circuit Using 16K 
CMOS RAMs with Two Chip-Enables 

For this example circuit, Figure 
9, when the system power is at its 
normal operating voltage, diode Dl 
and transistors TR1 and TR2 are all on 
(on conducting) , and the resulting 



VDD to the CMOS RAM is VS-VSAT2. 
VSAT2 is the saturation voltage of 
transistor TR2 and is typically 0.2V 
for common transistors. Referring 
to the previous example. Figure 8, 
the forward voltage drop of diode 
Dl, VF1, is typically in the range of 
0.6 - 0.8V, which is considerably 
higher than VSAT2._ Thus it can be 
seen that the circuit shown in Figure 
9 will deliver actual voltage to the 
CMOS RAM much closer to VS than in 
Figure 8. This difference is 
occasionally important to ensure the 
entire system works within the 
guaranteed voltage range. Guaranteed 
operational voltage range for systems 
using TTL or LSTTL gates if 5V+5% 
which CMOS RAMs usually allows 
5V+10%. Using a circuit similar to 
Figure 9 with VSAT2 0.2V, 5V+5% - 
VSAT still satisfies the RAM require- 
ment of 5V+10%. 

When the system power supply 
fails and the voltage starts to drop, 
the voltage from the backup battery 
is fed through diode D2 as soon as 
Dl, TR1 and TR2 turn off and the 
relationship VS < VBZ + VBEl is 
established. The voltage level at 
the memory during backup operation 
will be VDD=VB-VFZ. However, it is 
necessary, before VS < VBZ + VBEl is 
fully established, to make the Data 
Retention Control Signal, CDRC, a 
logical high and therefore place the 
memory system in the data retention 
mode. Figure 10 shows the data 
retention voltage characteristics 
of the TC5516 APL . 

It should be noted here that a 
low-power shottly gate (LSTTL) should 
not be used for the Data Retention 
Control Gate. In LSTTL circuits, 
the output terminals are connected to 
the power supply rail through an 
internal resistor which when used as 
the Data Retention Control Gate, will 
sink the battery current and degrade 
the life of the backup battery. 
Standard TTL gates and CMOS logic 
gates which have no such internal 
current path should be used. 

In Figure 9, the benefits of 
two chip-enables found in TC5516 APL 
and TC5518 APL have been fully 
utilized with CE2 commonly connected 
to all memory devices on the board 
through a single control line. 

The following are additional 

advantages users can expect with 
TC5516 APL and TC5518 BPL in battery 
applications . 


High speed memory systems can be 
implemented by using the fast 
access from chip-enable feature. 
Access time from CEl for the 
TC5516AP/APL is 100 nsec max. 


Both CEl and CE2 can be used to 
control the power down mode so 
that for large arrays, an X-Y 
decode arrangement can be used 
to control the active selection. 
This results in only one device 
being active at any time and 
therefore the average power con- 
sumption of the system is reduced 

Large Capacity Memory Application 

This example, Figure 11, shows 
the use of the two chip-enables of 
the TC5518BPL to make two-dimensional 
memory capacity expansion very simple. 

The configuration of the battery 
backup circuits and its operating 
principles are identical to those 
discussed earlier in Figure 9. The 
TC5518BPL is most suited for this type 
of system because the two chip-enable 
pins have identical functions and can 
be used interchangeable. Futhermore, 
the number of decoded circuits 
necessary to decode the multichip 
memory system is minimized in this 
2 dimensional memory array where 2n 
decoders can decode n2 memory chips. 

In an actual application where 
the memory system is used with a CPU 
whose data must be transferred and 
stored in memory when the power fails, 
it is necessary to detect the power 
failure as early as possible so that 
the DC voltage in the system is still 
sufficiently high to ensure accuracy 
during the memory write operations 
while VDD is falling. By utilizing 
power down voltage detection circuit 
which detects off the AC power supply 
line as is shown in Figure 12. The 
earliest warning of a power failure 
can be generated. This signal can 
be detected by the CPU resulting in 
the storage of critical data which 
VDD is still at its maximum, and then 



to generate the Data Retention Control 
signal which puts the memory system 
into the standby mode with battery 

When the memory system is used 
only as a ROM replacement and no 
writing to the memory occurs, the power 
fail signal, when detected, would 
simply place the system into the 
backup mode . 



Figure 2. Cross Section of CMOS 

The capacity of high- technology 
semiconductor memories, supported by 
improvements in finer, more advances, 
process technologies, has quadrupled 
every 2 years essentially without 
increasing the chip size of the devices. 
This indicated that the standby current 
of CMOS RAM will remain essentially 
constant even though the storage 
capacity of the devices will continue 
to increase. This fact and the basic 
benefit of low power dissipation of 
CMOS make the CMOS RAM increasingly 
more attractive for future applications, 
where low power and non-volatility are 
key issues. 

The low power performance of CMOS 
RAMs in the standby mode has improved 
to such a stage where even a small 
button type Lithium battery can be 
used to maintain data as long as 10 
years . 

New devices from Toshiba, TC556 4 
and TC5564, 64K Static CMOS RAMs, which 
are soon to be released should con- 
tribute heavily toward this direction 
in the future. 


Figure 3. Circuit Diagram of CMOS 

I DDS2 vs. VdD 








Ta = 25'C 
CE 2 =V DD-0.5V 


3 4 5 6 

V DD (V) 

Figure 4. Standby Current vs. VDD 



Figure 1. Circuit Diagram of CMOS 
Memory Cell 



IDDS2 vs. Ta 













v D D = 5.5V 

CE2 = 5.0V 

A — 


lPv ia ry 

II il o || 

a <\j ^ 

> O O Ft 




Figure 5. Standby Current vs. Ta 

m in 
jr <_» 

!3 - 

Figure 6. TC5516 APL Standby Current 
(IDDS2) Distribution Data 





MM , , , 

III 1 1 1 1 1 

null i i 

in 1 1 l i i 

(sunoH) J 


Figure 7. Data Retention Time vs. 
System Size 

Power (V s ) 

Figure 8. 






Li thf urn Battery 


Basic Circuit for Battery 
Backup Application 



Power , 

K r 1 r- 

1 [p\ 




Figure 12. Battery Backup Circuit with 
Power Down Signal 

Figure 9. Simple Application Circuit 
for Battery Backup 

Data Retention Period 

Figure 10. 


(1 merory cycle time 
tirre rain.) 

Address A, 3 , A, 4 

1/2 TC40H139P 

All .*12 

1/2 TC40H139P 

Figure 11. Large Capacity Memory 


18 pin 

24 pin 

Ce rd i p 



Address latch 
and synchronous 















TC5504APL | 4,096 x 1 









v ca 

2 2 




John L. Stegman, Jr. 
Electronic Design Engineer 
Catalyst Research Corporation 

1421 Clarkview Road 
Baltimore, Maryland 21209 


Applications of batteries for con- 
tinuous long-term memory backup are not 
as easy or as simple as one might first 
assume. It requires consideration of 
not only the battery but also the type 
of switch to be used to turn the bat- 
tery on and off, and the interface be- 
tween the memory (or microprocessor) 
and those components not under battery 
power . 

We have chosen a long-life, solid- 
state, lithium-iodine battery which has 
been used in the cardiac pacemaker in- 
dustry since 1972. Today, this battery 
powers more than 90% of the world's 
pacemakers, and its reliability is well 
documented . 

We have also designed and tested 
the switching, control and interface 
electronics needed for reliable data 
retention under battery operation, and 
during the transistions to and from the 
normal (DC) power supply. 


The battery is a solid-state, high 
energy density, long-life primary cell. 
It uses lithium metal as the anode, a 
solid pellet of iodine imbedded in an 
organic polymer as the cathode, and 
solid lithium iodide as the electrolyte 
and separator. It cannot suffer from 
separator rupture or loss of electro- 
lyte, which are common failure mecha- 
nisms in most batteries, because the 
separator /electrolyte is solid and self- 

The open circuit cell voltage is 
2.793 volts at 25 C and at end-of-life 
is only 3 millivolts less. The temper- 
ature coefficient of open circuit volt- 
age is about +1 millivolt per degree 
Celsius. Most CMOS memories will re- 
tain data at 2.0 volts, leaving almost 
0.8 volts for loss in the battery due 
to polarization, temperature change and 
voltage loss in external circuits. 

The cell current available is 
limited by the internal impedance of 
the battery and the minimum operating 
voltage required by the device under 
battery power. 

The cell impedance varies with 
capacity extracted and the cell temper- 
ature. For a given capacity extracted, 
cell impedance is essentially indepen- 
dent of the drain rate at which the 
cell had been used. Practically, this 
means the impedance is constant and 
predictable . 

The available cell current, at a 
fixed voltage, increases with increas- 
ing temperature and decreases with de- 
creasing temperature. The apparent 
cell impedance also varies with temper- 
ature, decreasing with increasing 
temperature . 

Self-discharge is the electrochem- 
ical process in which battery energy 
is lost internally without doing any 
external work. The extent to which 
this process occurs in the lithium-io- 
dine system is about 10% of capacity 
remaining over 10 years at 25 C. This 
is what makes the battery desirable 
for long-term use; most batteries will 
completely self-discharge in much less 
time . 

Lithium-iodine cells can accept 
reverse currents both continuously and 
momentarily, provided the magnitude of 
the reverse current is kept below the 
limit that causes damage to the cell. 
A small reverse current of about 1 
microampere will not adversely affect 
the battery. Preliminary tests at 
Catalyst Research indicate that these 
small reverse currents may prolong the 
life of the battery. It should not be 
assumed that the battery is being re- 
charged by these currents; the capacity 
of the battery is not significantly 
increased. Momentary reverse currents 
should be kept below 1 milliampere for 
periods of 1 second since excessive re- 
verse current can cause permanent cell 



damage . 


To readily interface the battery to 
a CMOS device requires the use of a 
switch to select battery or normal sys- 
tem power. The switch can be modelled 
as two single-pole single-throw switch- 
es with a common terminal. This con- 
figuration is necessary for make-before- 
break operation. The battery switch 
must operate under battery power, and 
need only pass small (<100 microamperes) 
currents. The other switch, for the 
normal supply, must pass sufficient cur- 
rent to operate the CMOS device in worst 
case situations without excessive loss 
of voltage. In addition, the normal 
supply switch must not present a load to 
the battery when the normal supply is 

battery's internal impedance; in the 
off-state the channel resistance is 
almost infinite . Because the bias cur- 
rent for a FET is so small it uses no 
appreciable amount of the battery en- 
ergy. There is some difficulty in 
using a FET since the gate-source bias 
voltage is at most the battery's open- 
circuit voltage, but by proper select- 
ion a suitable device may be found. 






9 1 


The Battery Switch 

The battery switch can be a sili- 
con diode, a germanium diode, or a field 
effect transistor (Fig. 1A,B). A relay 
may be used although its reliability 
over the life of the product must be 
taken into consideration. 

The silicon diode is inexpensive 
and easy to- use. The primary disadvant- 
age of the silicon diode is the forward 
voltage drop. Although this drop may* 
be as high as 0.7 volts at moderate 
currents, tests we have conducted show 
that a type 1N4148 diode at a forward 
current of 10 microamperes has a volt- 
age drop of only 0.35 volts at 25 C. 

The germanium diode is also an 
attractive switching device for the bat- 
tery. Its forward voltage drop is much 
lower than that for a silicon diode. 
For a type 1N34A germanium siqnal diode 
at 10 microamperes forward current, the 
voltaqe drop is less than 100 microvolts 
at 25 C. However, because of the maqni- 
tude of the reverse bias current in qer- 
manium diodes, the battery may see 
siqnificant reverse current under normal 
supply ooeration. Small reverse cur- 
rents present no difficulty provided 
these currents do not exceed the capa- 
bility of the battery. 

The field-effect transistor has 
shown to be an almost ideal device for 
this application. In its on-state, the 
channel resistance is much less than the 

Fig. 1(A) Fig. 1(B) 

The Normal Supply Switch 

The normal supply switch must pass 
sufficient current to operate the CMOS 
device without voltage loss great 
enough to cause logic level conflict. 
For CMOS devices this loss cannot ex- 
ceed 0.3 volts, typically. The normal 
supply switch must also not load the 
battery when the CMOS device is under 
battery power. Two possible choices 
for the normal supply switch are a 
diode and a bipolar transistor (Fig. 
2A,B) . 

The silicon diode is inexpensive 
and easy to use but its forward volt- 
age drop in this application will be 
0.7 volts because of the current re- 
quired under normal operation. To 
prevent logic-level incompatibility, a 
separate supply voltage may be needed 
and a bias resistor to hold the diode 
drop at 0.7 volts at all times. The 
reverse bias current thru a low-leak- 
age silicon diode is small enough that 
it will not load the battery during 
battery operation. A germanium diode 
would load the battery, and should not 
be used here. 

The PNP-bipolar transistor is a 
very attractive switch in this appli- 
cation. A low emitter-collector satu- 
ration voltage and low reverse current 
thru the collector-base junction meets 
the requirements of the normal supply 



switch. Suitable transistors are avail- 
able for a wide range of normal supply 
currents. Caution must be exercised to 
avoid forward biasing the collector- 
base junction, and thereby loading the 
battery, during battery operation. 



Fig. 2(A) 

Fig. 2(B) 

Transitions in Power Supplies 

The steady-state operation of the 
switch, either in battery mode or in 
normal supply mode, is easily accom- 
plished with combinations of solid- 
state devices as described. It is 
during the transition states, from 
battery to normal and normal to battery, 
that difficulties arise. 

It is obvious that even short dur- 
ation losses of power to a CMOS device 
will cause errors in the data contained 
in the device. Practically, this means 
that during power-up, the normal switch 
must close before the battery switch 
opens; and during power-down the bat- 
tery switch must close before the nor- 
mal switch opens. If a common signal 
is used to activate the changeover in 
power supply, the response of the 
switches must be guaranteed to meet 
this condition. 

Lithium-iodine cells can accept 
reverse currents both continuously and 
momentarily, provided the magnitude of 
the reverse current is kept below the 
limit that causes damage to the cell. 
A continuous small reverse current of 
about 1 microampere will not adversely 
affect the battery. Preliminary tests 
at Catalyst Research indicate that 
these small reverse currents may pro- 
long the life of the battery. It 
should not be assumed that the battery 
is beinq recharged by these currents; 
the capacity of the battery is not sig- 
nificantly increased. Momentary re- 
verse currents should be kept below 1 
milliampere for periods of 1 second or 
permanent cell damage can occur. 


The control mechanism must deter- 
mine power supply status and generate 
signals to switch between normal supply 
and battery supply. To facilitate the 
use of simple interfaces, the change 
from normal supply to battery supply 
should occur at about the nominal cell 
voltage; switching at this voltage im- 
plies logic-level compatibility during 
changes in power supply. A signal to 
indicate the mode of control of the 
memory (or microprocessor) must be gen- 
erated to determine if access will be 
permitted by the system hardware; under 
battery power the memory (or micropro- 
cessor) must not be accessed because of 
excessive demand on the battery. 

Power-Up/Down Sequences 

In a typical application, a normal 
operation sequence consists of : power- 
up, operation on normal supply, and 
power-down. It is essential that the 
data retention system be capable of 
handling the shortest length sequence; 
the length of this sequence is depend- 
ent on particular normal power supply. 
Abnormal power supply situations must 
also be kept in mind to assure data re- 
tention during these times. 

The power-up sequence can be ap- 
proximated by a smooth rise from zero 
volts to the normal supply voltage. 
When the normal supply rises to the 
nominal cell voltage, the change to 
the normal power supply should occur. 
When the normal supply rises to the min- 
imum DC operating voltage for the system 
hardware, access to the control of the 



memory (or microprocessor) may be per- 

A power-down sequence can be ap- 
proximated by a smooth drop from the 
normal supply voltage to zero volts. 
When the supply drops below the minimum 
DC operating voltage for the system 
hardware, access should be denied to 
the memory (or microprocessor) . When 
the normal supply voltage falls to the 
battery nominal cell voltage, the change 
should be made to the battery from the 
normal supply. 

This technique of two level volt- 
age detection will protect the memory 
(or microprocessor) hardware from most 
types of power supply failure and nor- 
mal on/off use. The use of hysteresis 
at the critical voltage detectors will 
further enhance the hardware protect- 
ion, designing fast power supply switch- 
es, characteristics of which were de- 
scribed before, will enable the hardware 
to handle abnormal power supply fluctu- 
ations with reliable data retention. 

False-Write and Read Protection 

A matter of great concern in bat- 
tery backup is the prevention of a 
"f alse-write" . A false-write is any 
attempt: (1) To write into memory data 
of questionable validity, or (2) The 
premature termination of a normal write 
cycle. The first case of false-write 
can be effectively prevented by in- 
hibiting operation of the external hard- 
ware whenever the normal power supply 
voltage is below minimum DC operating 
voltage, and granting access on a power- 
up sequence only after establishing 
normal system operation. The second 
type of false-write, premature termi- 
nation, occurs during a power fail se- 
quence. Premature termination is pre- 
vented by sensing the impending loss of 
power before minimum DC operating volt- 
age is lost, and halting system oper- 
ation before insufficient voltage is 
available to complete the write cycle. 

The power required for a normal 
read of memory usually exceeds the capa- 
bility of the battery. Pead access 
must be prevented during battery oper- 
ation or loss of data may occur due to 
a momentary drop of battery voltage be- 
low the minimum data retention voltage. 


The digital interfaces to battery- 
backed memory (or microprocessors) can 
be divided into three classes: Control 
signals, address signals, and data sig- 
nals. Control signals present the most 
difficult problem in interfacing be- 
cause the interface must take over con- 
trol of the device during battery op- 
eration. Because the battery cannot 
supply enough energy for normal oper- 
ation, the change in control must pre- 
cede the change to the battery supply, 
and follow the change to the normal 
supply . 

Control Signals 

Control signals asserted low (neg- 
ative) must be pulled high during bat- 
tery operation. We have developed and 
tested a simple and reliable interface 
circuit that performs this function 
(Fig. 3A) . A type 74LS03 open-collect- 
or, dual-input nand gate has the follow- 
ing property: If one of its inputs is 
held at ground potential, then its out- 
put transistor will remain completely 
off (not conducting) for any voltage 
at the Vcc terminal greater than zero 
volts and less than +5 volts. This 
property has been theoretically and 
empirically verified for several major 
manufacturers (Tx Inst., Natl Semi, 
others) . This property has not been 
checked for more sophisticated LS-TTL 
devices; it is the simplicity of the 
74LS03 internal construction that gives 
it this property. Therefore, a pull- 
up resistor connected to the CMOS 
memory (or microprocessor) Vdd terminal 
will hold the CMOS input connected to 
the 74LS03 output at cmos Vdd during 
the rise and fall of the normal power 
supply voltage. The signal that is 
held at ground potential at the nand 
gate input we call "Access Grant", for 
obvious reasons. The remaining nand 
gate input is the system control input 
with inverted polarity; we call this 
"Access Request." It is much easier to 
generate an access grant signal held at 
ground than to attempt to disconnect 
the CMOS input from the system control 
output with a switch. There is no 
need to be concerned with the access 
request line; it can fall to ground in 
any manner provided its voltage does 
not exceed the voltage at the Vcc term- 



inal of the 74LS03. 

Address Signals 

Control signals asserted high 
(positive) present a somewhat less 
difficult problem, since the CMOS in- 
put must be held at ground. Using any 
open-output (collector or drain) device 
such as gates, comparators, or more 
sophisticated function devices, and 
connecting the pull-up resistor to a 
switch that turns the pull-up into a 
pull-down (to ground) will hold the 
CMOS input at ground. (Fig. 3B) is an 
example of this technique using a 74LS 
05 open-collector inverter. Designing 
the appropriate switch for the pull-up/ 
down resistor is an easier task than 
attempting to disconnect the CMOS in- 
put from the system control input. 
(Note: Analog switches usually do not 
function correctly during their own 
power-up/down, and are unreliable in- 
terface devices.) 


Fig. 3(A) 

Fig. 3(B) 

The problem of address signal 
interfacing is essentially one of 
avoiding voltages at the CMOS address 
inputs that fall in the CMOS input 
"Linear Region." This linear region is 
between the maximum input-low voltage 
and minimum input-high voltage. Input 
voltages in this interval cause both 
the N-channel and P-channel transistors 
to conduct heavily. This current is 
pulled through the power supply inputs; 
if the CMOS device is under battery 
power then the battery will attempt 
to source this current. Not only does 
this condition waste battery power, if 
it occurs simultaneously on many inputs 
it will saturate the battery and cause 
the cell voltage to drop below the min- 
imum data retention voltage for the 
memory (or microprocessor) , and the loss 
of data. This problem is solved by 
holding the address inputs at ground 
potential during battery operation. A 
careful choice of address bus-driver/ 
interface will facilitate this. In 
any case, an interface like those for 
control signals described above will 
also work for address signals. 

Data Signals 

Data signals are the easiest to 
interface. If the data in/out to the 
memory (or microprocessor) is of the 
three-state bidirectional type then 
no interface is required if the data 
in/out terminals are in the high imped- 
ance state. The particular memory (or 
microprocessor) should be checked for 
excessive leakage current in the high- 
impedance state. Memory (or micropro- 
cessor) devices with separate data in- 
puts and outputs can be treated like 
address inputs for data inputs; and 
data outputs should be disconnected 
from their loads, either internally or 
externally . 


The use of lithium-iodine batter- 
ies for long-term, uninterruptable 
power supplies for CMOS memory (or 
microprocessors) is quite feasible. 
Their applications in products can be 
separated into problems independent of 
each other, and the solutions of the 
problems can be integrated to form a 
highly reliable system without the use 
of expensive or dif f icult-to-f ind de- 
vices. We have developed a series of 



successful products based on the lith- 
ium-iodine battery; in each case the 
probability of lost data was reduced 
to the probability of failure in the 
actual memory (or microprocessor) in 
the product. 

Note : 

The author would like to thank 
Alan Schneider for his guidance in 
the preparation of the manuscript.