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Battery Management - Gas Gauge Forum
BQ76925 - Voltage regulator shutdown without using I2C
The BQ76925 AFE part has an internal LDO that supplies power to the part directly from the battery.This is a problem when the host microprocessor has a separate power supply that is switched off without the host first signaling to the AFE using I2C to shutdown. In this case the AFE draws an excessive current (~3mA) continuously from the battery while the host is off.(This is because it is trying to provide an analog output voltage on some pins to the powered-down host.)I discovered that by leaving the VCTL pin floating, and connecting the host's VCC to V3P3, the part appears to work ok and the part's internal LDO appears to shut down when the host VCC is off and so draw an acceptably low current (<10uA) even when the part is not shut down over the I2C.This will be necessary in many applications like mine where the host microprocessor has a separate power supply, but cannot 100% guarantee that the BQ76925 will get sent the shutdown sequence when the host loses power.Does anybody know if this is an acceptable use of the BQ76925? What could go wrong?
An expected operation of the '925 is to have its regulator provide power to the micro-controller and the micro-controller put the 925 to sleep after turning off appropriate outputs. Wakeup requires signaling on the ALERT pin, which would not be possible from the shutdown micro-controller but would have to come from some other source.
The LDO (V3P3) may be back-fed by an external source as described in the datasheet. If the supplied voltage is higher than the internal regulator, the internal regulator provides no current. If the supplied voltage is below the internal regulator, the internal regulator attempts to provide current to raise the voltage. Leaving the VCTL pin un-connected should not damage the part as long as no abs max value is exceeded, but the recommended operation would be to have a large value pull-up resistor from the VCTL pin to the BAT pin. This should limit the current delivered by the internal regulator to a very low level.
When providing a switched regulated voltage to the V3P3 pin, the micro-controller should not put the '925 in sleep mode if the same regulated voltage is intended to wake the part from the ALERT pin since the part will wait for the voltage to fall near 0V before accepting the ALERT pin wake signal.
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