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BQ25172: STATUS Output Faster Than 1Hz

Sam Russell
Prodigy 10 points
Part Number: BQ25172

Tool/software:

We are using the BQ25172 to intermittently charge a 4-cell NiMH battery pack for 12 hours at 120mA. The VBAT status is connected to an LED and an input to a microprocessor (which is configured as an input), the STATUS pin/LED works as expected when charging and fully charged cell is connected.

However during open circuit (or I assume any fault condition) the STATUS pin is driving the LED more than once a second, the LED is blinking roughly twice then three times a second when using a 4.7uF output capacitor and no connected battery, this then increases to decreases to roughly twice a second with 22uF output capacitance and remains on with no output capacitance. What is the reason for this behaviour? attached schematic below, the left hand side is the supply to the BMS and the right is to the battery, the output of the charger is configured to discharge only when the input side voltage has dropped below ~10V.

The EVK does not display this behaviour with a 1uF output capacitance.

 

  • 0
    TI__Guru* 75656 points

    Hi Sam

    This is normal behavior for a no battery condition.

    Sam Russell said:
    What is the reason for this behaviour?

    When powered on the BQ25172 will charge output capacitor to OVP threshold, 1.7V / cell.  Fault will be set, charger turns off and STAT indicated a fault.  Voltage on the capacitor will decrease due to load and at 1.33V / cell unit will start and intermittent charge cycle.  Unit will enter OVP again and repeat this sequence. 

    Sam Russell said:
    However during open circuit (or I assume any fault condition) the STATUS pin is driving the LED more than once a second, the LED is blinking roughly twice then three times a second when using a 4.7uF output capacitor and no connected battery, this then increases to decreases to roughly twice a second with 22uF output capacitance and remains on with no output capacitance.

    The large the output capacitor the longer the discharge time.