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BQ24725A: BQ24725A ACFET unexpected switched off

Peng Liu50174
Expert 1510 points
Part Number: BQ24725A

Hi, Experts,

We are experiencing one unexpected phenomenon randomly with BQ24725ARG----ACFET (Q27 in the attached schematic) is switched off when both AC (ACOK voltage is 3.35V) and battery are connected, the battery is being discharged continuously until low voltage to shut off the system. We have to remove both AC and battery to make ACFET back ON.

The capacitors value are not the same as that in the EVM schematic, can these parameters deviations cause charger to switch off ACFET? Or is there any other situation such as transient event which can cause charger to switch off ACFET and never comes back again?

Below are some questions regarded the parameters used on our board, hope get your comments.

1), 33nF C291 is installed. Does it have negative impact?

2), What is side effect of installation of C379 and R548?

3), How to choose C470 and C548?

The schematic pictures are shown below.

Thanks,

Peng,

  • 0
    Expert 1510 points

    The schematic pasted in the original post is gone, re-attached here.

  • 0 in reply to Peng Liu50174
    TI__Genius 11850 points

    Hi Peng,

    1) I recommend removing the 33nF C291 capacitor. The reason for this is to follow the Cgs and Cgd selection guidelines given in 3).

    2) C379 can help with common-mode filtering on SRP, but it is not required. R548 can help discharge the BATFET's Cgs more quickly. C379 and R548 are not populated on the EVM, but I don't think they would cause the ACFET behavior that you observe.

    3) The Cgs and Cgd capacitor selection is discussed at the bottom of page 17 in the datasheet. For reference, the BQ24725A EVM uses Cgs = 100nF and Cgd = 2.2nF. In general, I suggest following the EVM component values.

    There are several conditions which can latch off the ACFET and RBFET. Before examining these conditions in more detail, I recommend updating the capacitors on your board so that they are more similar to the BQ24725A EVM.

    Best regards,

    Angelo