RECOMMENDED: This section includes a few recommendations when specifying the battery and battery life.
1.Although it is common to derating batteries by 25% from their specified capacity quoted at 25°C, a derating factor of at least 50% is recommended by Xilinx.
2.The total power in the battery-powered domain, which includes both RTC and BBRAM, is expected to be 2.5 µA at 50°C, with the IBATT consumed by the BBRAM as specified in Zynq UltraScale+ MPSoC Data Sheet: DC and AC Switching Characteristics (DS925) [Ref 2]. Since it is not possible to power off the BBRAM, this leakage must be included when calculating battery life.
3.Power consumption in the RTC and BBRAM is dominated by leakage; therefore, using leakage as the only source of power consumption gives an accurate estimate of the battery life.
4.A leakage requirement of 2.5 µA for the battery-powered domain is specified at 50°C, which is more pessimistic than 25°C. Despite this, the same requirement is used at 25°C. This temperature (25°C) is a typical specification for battery life.
5.Battery consumption in the battery-powered domain is limited to when the PS main supplies are off (including VCC_PSAUX). Since the PS is never completely off (most of the time, although it could be in deep-sleep mode), the battery life (in years) can be divided by the percentage of the time the device is used, to get the number of years the battery should last. Embedded systems are rarely completely off and the need to turn off the device is even less in Zynq\ UltraScale+ MPSoCs due to the availability of the deep-sleep mode.
6.Xilinx recommends using batteries in the specified range of the VCC_PSBATT voltage (1.2V-1.5V). Using batteries with voltages higher than 1.5V requires a low dropout regulator (LDO) or voltage divider. Although LDOs and voltage dividers cause more current to be drawn from the battery even during PS power up.
Assuming an average current of 2.5 µA is required by the BPD and 50% derating on the battery, a 438 mA-hour battery is required to sustain 10 years of continuous operation (see This Equation).
Equation 7-3 2.5 µA x 1 mA/1000 µA x 10 years x 8760 h/1 year = 219 mA-h/50% derate = 438 mA-h
Assuming a 33% system off time (using the battery), the system can operate for 10 years with one 146 mA-h battery using a 50% derating factor. Table: Battery Lifetime for BPD (using Example Battery Types) shows the lifetime of a battery depending upon the battery chosen to power the BPD.