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BQ25756: after the completed charging, the charging state is abnormal

s sun
Prodigy 170 points
Part Number: BQ25756

Tool/software:

Hi,dear.

After two hardware iterations, the charger worked perfectly upon powering on, i.e. the charger can normally charge the battery every time and the red led is on continously during charging. 

But after the end phase of charging (the charging current decrease to about 0.1A or below, actually my design current is set to about 2.4A), the two leds which seperately connected to the chip pin STAT1(red led) and STAT2 (green led) will be on in turn continously,  i.e. red led is on then off and green led is off then on every 1S or below. 

The normal situation is that after the completed charging the green led is on continously.

How to solve it?

Thank you!

  • 0
    TI__Expert 6755 points

    Hi,

    Is there a battery connected after the end phase of charging? This type of behavior normally happens when the output of the circuit is left open. The capacitors charge up, reach termination, self-discharge, and then recharge after reaching the threshold (default 97.4% of the termination voltage).

    If there is a battery still connected, can I take a look at the schematic?

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 170 points

    HI,dear.

    The battery is still connected.

    The attachment is my schematics.

    Thank you!

    CHARGER-v12.pdf

  • 0 in reply to s sun
    TI__Expert 6755 points

    Hi,

    Thanks for sharing the schematic. I think the LM74700 devices are blocking the battery in this instance and causing the capacitor behavior I mentioned earlier. Can you provide a waveform with the node labeled BAT_TEMP and the node labeled PACK+? I believe that PACK+ is the battery output here, correct?

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 170 points

    Hi, dear.

    In order to prevent the spark from damaging the charger chip when the battery is connected to the board, i add the ideal diode between the output of chip and the battery.

    Actually the PACK+ is the battery input which is the anode of battery.

    Can you provide a waveform with the node labeled BAT_TEMP and the node labeled PACK+? ---------My understanding is that I should measure the waveform with the node labeled BAT_TEMP and the node labeled PACK+ when the battery is full, i.e. the two leds are on in turn. Is my understanding right?

    Thank you! 

  • 0 in reply to s sun
    TI__Expert 6755 points

    Hi,

    Thanks for the clarification. It makes sense to have that kind of ideal diode protection here.

    Yes, please provide those waveforms while the battery is full and the two LEDs are switching on and off.

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 170 points

    hi,dear.

    I remove the ideal diodes where is between the output of charger chip and the battery connector. The charger chip work well and the led show normal, i.e.when the battery is full, the green led (STAT2) is always on.

  • 0 in reply to s sun
    TI__Expert 6755 points

    Hi,

    I am glad to hear that it is working! I am not sure that we can prevent the STAT1 and STAT2 oscillation with the ideal diode. If you have an MCU you might be able to use the FORCE_STATx_ON bits once the charge has reached termination.

    Best regards,
    Michael Bradbourne