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BQ25703AEVM-732: Power input to 3.3v LM3480

Part Number: BQ25703AEVM-732
Other Parts Discussed in Thread: LM3480, BQ25703

Hello, I hope you are well,

I am working on a custom PCB based on the BQ25703A-EVM, and I am a little confused about the power input to the 3.3v LM3480 LDO. According to the schematic: the LDO is powered from either VBAT or VBUS, through one of two diodes connected in a common cathode configuration. As far as I can tell this LDO must be powered up in order for the BQ25703 to begin charging, as it is required to pull up the chrg_ok net, which turns on the input FETs Q5 and Q6.

What confuses me is that if the battery is removed, or if there is a BMS interrupting this battery connection due to low battery voltage, there seems to be no way for the device to power up because the VBUS net is downstream of the input FETs. There seems to be a 'chicken or egg' issue, where the device needs to be powered already in order to accept the power it needs to run.

Is this desired behavior? To me it makes more sense for the LM3480 to get power from the VIN net. The LM3480 can accept up to 30 volts at it's input, which should keep it safe from any power source remotely resembling USB-PD, and getting power from VIN, as opposed to VBUS, would allow the device to wake up from adapter power.

Thanks for your time and support,


  • Hi Tim,

    If no power is applied at 3V3 net when adapter is attached, Q5 backgate diode will conduct and R14 will pull Q6 gate voltage to GND (overcoming the 100K resistor at R9), which will close the FET. (Note that Q5 and Q6 are PFETs as opposed to the NFETs that are used for Buck/Boost switches at Q1-Q4. )

    When 3V3 is available, asserting CHG_OK will divert 3V3, turning off Q9A and Q9B so that R14 will continue to overcome R9 to allow current flow. When CHG_OK goes low, this will route 3V3 current through Q9A, which will turn on Q9B to short out the 100K R9. This short will overcome the R14 pull-down, bringing the Q5/Q6 gate voltages up to the common node voltage, turning them off.