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Battery technology used in NVRAM products from ST

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AN1011 Application note
Batter y technology used in NVRAM products from ST
Lithium Carbon MonoFlouride (LiCFx) batteries are used by STMicroelectronics in their line of ZEROPOWER and TIMEKEEPER 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 2007

Rev 2

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www.st.com

Characterization and modeling

AN1011

Characterization 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:

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 (B) BR1632 discharge rate

3.5 Voltage (V) Voltage (V) 3.0 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

3.5 3.0 2.5 2.0 1.5 1.0 0 1000 2000 3000 4000 5000 6000 15k 30k 50k 100k

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 datasheet for each device.

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AN1011

UL 1950/60950 validation

UL 1950/60950 validation
All ZEROPOWER and TIMEKEEPER devices, with a self-contained lithium cell, are recognized by the Underwriters Laboratory Inc. UL 1950/60950 specification. The conditions for acceptance under this include the following: 1. 2. 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. 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 7 VDC. That the devices are intended to be mounted on a printed wiring board, flame rated to a minimum of 94 V-1. That the devices are provided with an appropriate on-chip, reverse current protection circuit. (See Figure 3 for the circuit diagram.)

3. 4.

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

Note:

RA is the ESD protection resistor and RB is the battery protection resistor required by UL.

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UL 1950/60950 validation

AN1011

If you have any questions or suggestions concerning the matters raised in this document, please contact STMicroelectronics at the following address: http://www.st.com/stonline/domains/suppor t/contact_suppor t.htm Please remember to include your name, company, location, telephone number and fax number.

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Revision history

Revision history
Table 1.
Date Dec-1998 28-Dec-2007

Document revision history
Revision 1 2 Initial release. Reformatted document; updated UL 1950/60950 validation and technical support URL. Changes

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AN1011

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