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BQ24780S: bq24780S battery charger doesn't work on some board

user1287186
Prodigy 80 points
Part Number: BQ24780S

Hi,

I've implemented the schematic of the bq234780's evaluation board on one of my design. Schematic wise not much change from the schematic I found in the sluuba6 document.

The major changes are the following :

  • transistor for ACFET & BAttery charger FETs (buck) are FDMC8032L
  • FET for connecting Battery node to VSYS is FDMA8051L
  • Inductor is a 15uH

One some board (65% of 100 total built), Everything is working fine and the Battery node is rising to 13.65V (U charge set in register) when the battery is disconnected. This means that the buck battery charger is working as expected.

On 35% of the board, the charger doesn't start leading to a HIDRV of 5.7V, a LODRV of 0V. In this case, at startup, we can see that the HIDRV start toggling but is limited to 5,7V and that it stop after some ms.... the LODRV never toggle. Board consumption is normal...

I read about the "layout consideration for short circuit" within the datasheet and I'm not sure if it's this phenomenon which is going on or not...

Layout wise, I've as much as possible copied the layout from the sluuba6 document and I've two separate trace for Vsys and V BAT...

Any help to investigate this would be welcom.

Kind Regards,

  • 0
    TI__Guru 56650 points
    To debug this, we have to know which protection turns off the converter.
    Please check ACOC, HI_FAULT and LO_FAULT. Please disable them or change the setting one by one until we find it.
    And then, check the HIDRV, LODRV and switching node signal to make sure HIFET and LOFET doesn't have shoot through and doesn't speed too much time to turn-on.
  • 0 in reply to Wang5577
    Prodigy 80 points

    Hi Wang,

    Thanks for the reply. I did turn on the IFAULT_HI bit (register 0x37h bit 7) and the result is that the chip is now turning off the ACFET transistor so my system is no longer powered. DOes this means something?

    Kind Regards,

  • 0 in reply to user1287186
    Prodigy 80 points
    Hi Wang,

    Also, when I do not set the bit 7 from the register 0x37, I have 5.7V pulse on the gate of the High buck FET, nothing on the gate of the LOW buck FET. In the mean time I have pulse of around 800mV on the switching node while the 24V (drain of the HIGH FET) stay stable. After some ms, the Gate of the HIGH FET is locked to 5.7V while the switching node of the buck is stuck to around 800mV... So the battery node is also at 800mV.

    I don't even understand how my HIGH FET has 5.7V on its gate, 800mV on its source and does not fully conduct... as the Vgsth is max 3V according to trhe datasheet...

    Kind Regards,
  • 0 in reply to user1287186
    TI__Guru 56650 points
    Yes. We have to figure out why is the HIGH FET not turned on with 5.7V on its gate? As you said, the Vgsth is max 3V according to the datasheet...
    When the output current is too low, the bq24780S charger will hold LODRV off except the refresh pulse when the bootstrap capacitor voltage is low.