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AN1011 APPLICATION NOTE
Battery Technology Used in NVRAM Products from ST
Lithium Carbon MonoFlouride (LiCFx) batteries are used by STMicroelectronics in their line of ZEROPOWER(R) and TIMEKEEPER(R) devices. These are used to provide the battery backup for the static RAM array, and to maintain the running of the real-time clock in the TIMEKEEPER devices. When the discharge rates are low, these non-rechargeable lithium batteries are capable of maintaining a highly reliable voltage level for many years. Moreover, when discharge rates are substantially higher, these batteries are capable of supplying a moderate or high continuous power, and are able to cope with widely varying intermediate pulses. Lithium batteries, therefore, have found use for a variety of applications that require battery backup. To be used in electronic components, these cells also need to be composed of constituents that are nontoxic, non-corrosive, and non-explosive. They also must be chemically and thermally stable before, during and after discharge. STMicroelectronics utilizes the 49 mAh BR1225X cell, and the 130 mAh BR1632 cell. The electrolyte of these cells (see Figure 1) is based on an organic solvent, instead of a corrosive alkaline or acidic solution found in most conventional batteries. This greatly improves the cell's leakage resistance, and guards against the negative effects caused by leakage. Figure 1. Cell Cross-Section
(-)
(+)
Cell Can Gasket CFx Cathode Anode Cap Lithium Anode
Current Cathode
Separator and Electrolyte
AI02531
December 1998
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AN1011 - APPLICATION NOTE
CHARACTERISATION AND MODELING STMicroelectronics has conducted extensive tests on these cells, a great proportion being destructive testing. Three main mechanisms of battery degradation were modeled, to establish the effects under various loads, temperatures (up to 85 C) and other environmental variables:
s s s
Active lifetime of the cell, as a result of the consumption of the lithium anode Shelf life (storage lifetime) of the cell, as a result of the electrolyte drying up (evaporation) Self-discharge, as a result of small leakage currents inherent to the battery chemistry
The weight loss proved to be a valuable measure of electrolyte evaporation. Self-discharge is specified by the battery manufacturer, from 0 C to 85 C, not to exceed 0.3% per year. Models were then developed to predict the rate of electrolyte loss and what effect encapsulation of the cell would have. Tests on the cells, when encapsulated in the epoxy used in the SNAPHAT process, have demonstrated a significant reduction in electrolyte evaporation. Figure 2. (A) BR1225X Discharge Rate
LOAD CHARACTERISTICS Temp: 20C
(B) BR1632 Discharge Rate
LOAD CHARACTERISTICS Temp: 20C
3.5 3.0 Voltage (V)
3.5 3.0 Voltage (V) 2.5 2.0 1.5
2.5 2.0 1.5 1.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Duration (Hrs.) (A) 15k 30k 100k
15k
30k
50k
100k
1.0
0
1000
2000
3000
4000
5000
6000
Duration (Hrs.) (B)
AI02519
Figure 2 shows the load discharge graphs taken at 20 C. As can be seen, both cells produce a nominal 3 V output with a flat discharge curve until the end of their effective lives, and so confirms that both are suitable for providing battery backup to low leakage CMOS SRAMs. The battery lifetime of individual ZEROPOWER and TIMEKEEPER devices, as described in AN1012, is a function of the current load, the battery selected and the system duty cycle. System life, which even with 100% duty cycle can still be greater than ten years, is specified in the data sheet for each device.
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AN1011 - APPLICATION NOTE
UL 1950 VALIDATION All ZEROPOWER and TIMEKEEPER devices, with a self-contained lithium cell, are recognised by the Underwriters Laboratory Inc. UL 1950 specification. The conditions for acceptance under this include the following: 1. That the devices are intended for use as components where the replacement of the battery can be performed by the operator or a trained technician. 2. That the devices are intended for use as components in low voltage, isolated, secondary circuits; where the case temperature does not exceed 100 C, and the voltage on any pin, relative to ground, does not exceed 7VDC. 3. That the devices are intended to be mounted on a printed wiring board, flame rated to a minimum of 94V-1. 4. That the devices are provided with an appropriate on-chip, reverse current protection circuit. (See Figure 3 for the circuit diagram.) Figure 3. Battery Control Circuit
VCC
P
(OFF when the VCC is below the battery voltage) VSO
RA BATTERY
RB INTERNAL VCC P
VSO
(ON when the VCC is below the battery voltage)
AI02520
Notes: 1. RA is the ESD protection resistor, and RB is the Battery protection resistor required by UL
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AN1011 - APPLICATION NOTE
If you have any questions or suggestions concerning the matters raised in this document, please send them to the following electronic mail addresses:
apps.nvram@st.com ask.memory@st.com
(for application support) (for general enquiries)
Please remember to include your name, company, location, telephone number and fax number.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. (c) 1998 STMicroelectronics - All Rights Reserved The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners. STMicroelectronics GROUP OF COMPANIES Australia - Brazil - China - France - Germany - Italy - Japan - Korea - Malaysia - Malta - Mexico - Morocco - The Netherlands - Singapore Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A. http://www.st.com
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