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BQ24780S: ACFET/RBFET latch-off condition

Tamio Yokokura
Expert 3960 points
Part Number: BQ24780S

As far as I see the datasheet of BQ24780S, there are the following two condition for being latching-off ACFET/RBFET.

  1. ACOC detection
  2. Seven times detection for 90sec that G-S voltage of SCFET/RBFET is less than 5.7V for 20msec after turning on FET

Is there other condition to latch-off ACFET/RBFET without the above two conditions?

As for Inductor and High-side/Low-side FET short circuit protection in buck mode, charger FET is latched off and both side FETs is turned off(not latched off).

Is this thought correct?

Thanks

Tamio

  • 0
    TI__Expert 5330 points

    Hello Tamio,

    Because the ACDRV is high (to turn on ACFET/RBFET) only whenever ACOK is high, any condition that causes ACOK to go low will also cause the ACDRV to go low and disable ACFET/RBFET.  So, for instance, if the ACDET drops below 2.4V, the ACOK will go low and as a result, the ACFET/RBFET will be turned off.  From datasheet 7.2 functional block diagram, ACOK can be deasserted due to VCC UVLO or ACOVP.  There is also a relastionshp between VCC and V_SRN shown in section 7.3.1.2 that can lead to ACOK deasserted.

    Regards,

    Steve

  • 0 in reply to Steve Preissig
    TI__Genius 16245 points

    Tamio,

    As Steve mentioned the rule of VCC and V_SRN, it is easy happened when customer design 4cell battery and each cell voltage is higher, may be around 4.4V/cell.

    Customer will easily see ACFET will be turn off during system dynamical load transient due to VCC voltage drops below (V(vcc-srn-fall) criteria.

    Just share my experience before.

    Thanks