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Original question:

BQ76952: SCD triggering issue

BQ76952: SCD behavior of BQ76952 at different current levels

Sunil Kumar
Prodigy 65 points
Part Number: BQ76952


Hi,

 We have an issue in SCD with BQ76952. When the voltage across SRN and SRP crosses 500mV then MOSFET switch off after 60us and then again switch ON after 6ms. Then switch off in 400us + set SCD delay. The SCD is triggered in AFE in the second case. Is there such a feature in which AFE behaves like this? In this image, Blue is the DSG gate to VSS and Yellow is SRN and SRP voltage. Mosfet once switched off the switched on again and then SCD is detected. Below 500mV everything is working fine.

The same image is zoomed in when MOSFET switches off for the second time.

Another test with higher current and zoom at when MOSFET switched off the first time.

Regards

Sunil Kumar

  • 0
    TI__Genius 11795 points

    Hey Sunil,

    I would imagine that an OCD fault is being triggered for the first event. What are your OCD1, 2, and 3 threshold, delay, and recovery settings?

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 65 points

    Hi, Caleb thanks for the reply.

    As for the first time current Fet switched off after only 60us, so it can not trigger OCD fault as for OCD fault minimum delay we can set is 1 i.e. 3.3ms but in this case, MOSFET triggers only after 60us. However, I have set OCD1 to 40 and the delay is 120 and OCD2 is 100 and the delay is 127. OCD3 is 40A and delay is 10s.

  • 0 in reply to Sunil Kumar
    TI__Genius 11795 points

    Hey Sunil,

    You're correct, the minimum settings for OCD is too large for it to be active in your case.

    The behavior is typical of a reset caused by SRP and SRN going high at the short circuit event. Be sure the design keeps SRP near VSS, recommended range is less than 0.75V.  A recent data sheet change added a description in the layout section for optional to add 0.1 uF caps to VSS near the IC.  This will help to avoid the reset if the design has large transients on SRP, SRN.

    If there are already caps, make sure that they are as close as possible to the SRP and SRN pins.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 65 points

    Hi, I have tested again with no noise on SRN and SRP pin but the behavior is same even when SRP and SRN voltage is less than 0.75 voltage.

    Here CH1 is Current(SRP) and CH2 is DSG Gate, with reference to VSS.

    I am waiting for your view on this.

    Regards

    Sunil

  • 0 in reply to Sunil Kumar
    TI__Genius 11795 points

    Hello,

    I will speak to my team and get back to you tomorrow.

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    TI__Genius 11795 points

    Hello Sunil,

    Where is the ground probe? During discharge the SRP pin should be at ground. If your SRP pin is at 800mV and you are discharging, then it's safe to assume that your SRN pin is at a higher voltage. If that's the case, then your part is probably still restarting.

    It would help to see a schematic. 

    Thanks,
    Caleb

  • 0 in reply to Caleb Perez
    Prodigy 65 points

    Sorry, CH1 proble of DSO is at SRP. Still I will check if there is any voltage difference between power GND and Digital GND

    Thanks

    Sunil