AN1012 datasheet(10/33 Pages) STMICROELECTRONICS

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Battery backup current - predicting data retention time

AN1012

10/33

Doc ID 6395 Rev 4

3.3

Capacity consumption

When VCC is being held by the external power supply within its specified range, the current

drawn from the battery is zero. When VCC falls below the battery backup switchover voltage

(VSO), the device goes into battery backup mode and draws all of its current from the

battery.

The VCC duty cycle represents the proportion of time, expressed as a percentage, that the

device is supplied with power from the external supply, and therefore not drawing current

from the battery.

In its battery backup mode, the array of SRAM cells can be characterized by its data

retention (ICCDR) current, caused primarily by the current through the Poly-R load resistors

in the 4T technology, as well as also by junction leakage, sub-threshold current, and gate-to-

substrate leakage. The total current is referred to as IBAT (the current drawn during battery

backup mode). For ZEROPOWER® devices, this is the sum of leakage currents plus the

current necessary to maintain the SRAM array. For TIMEKEEPER® devices, it is the sum of

the array current (including leakage) and the clock current:

IBAT = IARRAY + ICLOCK

Many factors need to be taken into account when calculating the IBAT current, including

process parameters, working temperature, and the VCC duty cycle.

3.4

Calculating capacity consumption

Capacity consumption is simply calculated by:

where:

●

Battery capacity is measured in ampere-hours;

●

8760 is the constant for the number of hours there are in a year;

●

VCC duty cycle is measured as a percentage; and

●

IBAT is measured in amperes.

For the M48T35Y, a 32K x 8 TIMEKEEPER® device with a 0.048 Ah (48 mAh) M4T28-

BR12SH1 battery, the typical battery current is approximately 2666 nA at 70 °C.

So, if the VCC duty cycle is 50%, the predicted capacity life is:

and therefore is about 4.11 years at 70 °C.

BatteryCapacity

8760

CCDutyCycle 100

⁄

–

() I

BAT

-----------------------------------------------------------------------------------------------------

0.048

8760

0.5

2666

10 9

–

-------------------------------------------------------------

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