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BQ24074: disconnecting the input voltage with simulated battery

Part Number: BQ24074

Dear expert,

My customer is using BQ24074 and meet some problem when disconnecting the input voltage with simulated battery.

The test method is as follows:

1. VBAT is connected to a constant voltage source, which can simulate the battery, and voltage is set to 3, 3.5, 4, 4.15V.

2. VIN_CHG is connected to constant voltage input, and the voltage value is set at 6.7V.

3. After connecting the circuit, the constant voltage source displays a negative current, indicating that charging has begun. At this time, disconnect the VIN_CHG input and measure the voltage at the VIN_CHG terminal.

Test Results: When the voltage of the simulated battery voltage source is 3.5, 4, 4.15V, after the input is disconnected, the voltage of 3.5, 4, 4.15V (corresponding to the voltage of the simulated battery) appears at the input terminal. When the simulated battery voltage is 3V, the above situation does not occur.

Could you help to support? Thanks.

  • Hello,

    This is because of the input capacitor on the VIN_CHG side. The  input FET is turned on when VIN is above VBAT + VIN(DT) which is the detection threshold of about 80mV. This means that the FET is on until the voltage goes below that threshold. WHen VIN_CHG is removed, the capacitor is discharged to roughly VBAT voltage and then holds that voltage until it gradually self discharges. You can try discharging the input capacitor with a resistor after you remove VIN.