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bq24725A - ACOK dropping and can't figure out why

Tim Anderson76
Prodigy 130 points

I have a charger replicated multiple times on one PCB. These charger blocks have all been tested functional on one PCB. On follow on PCBs, random charger blocks are not working. So I am trying to debug these boards to see why the charger blocks are not working. Is my design marginal, is there a component error, a soldering issue, etc...

On one charger, I have gotten to the point where I can see the ACOK pull down for something like 150-175ms and then come back up. This condition appears to happen as the charger attempts to start charging. As I can send "charger current" and "charger voltage" commands via smbus, and that will trigger this event. In other words, this is very repeatable.

I can find some ACOK requirements in the datasheet:

"AC adapter detection open drain output. It is pulled HIGH to external pull-up supply rail by external pull-up resistor when voltage on ACDET pin is between 2.4V and 3.15V, and voltage on VCC is above UVLO and voltage on VCC pin is 275mV above voltage on SRN pin, indicating a valid adapter is present to start charge. If any one of the above conditions can not meet, it is pulled LOW to GND by internal MOSFET. Connect a 10kΩ pull up resistor from ACOK to the pull-up supply rail."

So I have been testing each condition when the ACOK gets pulled low. ACDET is a solid 2.6V. Vcc drops maybe a volt (from 20.8 to 19.8V). Which is still way above UVLO of 4V. And SRN starts to rise before ACOK gets pulled low, but only reaches ~2.8V. So Vcc-SRN is roughly 17V, much higher than 275mV.

So all three conditions for ACOK to be high are met, and yet ACOK is pulling low. Are there any other hidden conditions that I can check?

Here is a plot. 

Ch1 (yellow): Vcc; Ch2 (blue): SRN; Ch3 (pink): 3.3V; Ch4 (green): ACOK; MATH (red): Vcc-SRN

I have also looked at the Iout pin in both input and output current mode, and see no current spikes.

What am I missing? Why is ACOK pulling low?

Thanks

  • 0
    Prodigy 130 points

    Updates:

    I see the Hi gate switching the Hi FET. I see the Phase node start switching accordingly. The Hi gate stops switching when ACOK goes low.

    CH3: Hi FET Gate

    I also see REGN start to droop. RGEN starts at a solid 6V. the Hi fet starts switching. I see some small spikes on the LO fet drive pin (~1.5V), and after a time, REGN starts to drop, when it reached ~4V, the ACOK goes low. So this may be the reason why ACOK is dropping, but now I need to know why REGN is dropping.

    .

    CH3: REGN

  • 0 in reply to Tim Anderson76
    Prodigy 130 points

    It looks like the low side FET was not properly soldered. Too much solder under the drain was causing the part to lift, causing the source pins to be just barely connected with solder. Once the charger tried to source current, the connection had too much resistance.