Data Sheet

Battery backup current - predicting data retention time AN1012
10/33 Doc ID 6395 Rev 4
3.3 Capacity consumption
When V
CC
is being held by the external power supply within its specified range, the current
drawn from the battery is zero. When V
CC
falls below the battery backup switchover voltage
(V
SO
), the device goes into battery backup mode and draws all of its current from the
battery.
The V
CC
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 (I
CCDR
) 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 I
BAT
(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:
I
BAT
= I
ARRAY
+ I
CLOCK
Many factors need to be taken into account when calculating the I
BAT
current, including
process parameters, working temperature, and the V
CC
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;
● V
CC
duty cycle is measured as a percentage; and
● I
BAT
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 V
CC
duty cycle is 50%, the predicted capacity life is:
and therefore is about 4.11 years at 70 °C.
BatteryCapacity
8760 1 V
CC
DutyCycle 100⁄–()I×
BAT
×
-----------------------------------------------------------------------------------------------------
0.048
8760 0.5 2666 10
9–
×××
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