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bq76940 - CHG_ON and DSG_ON failed to turn on

Tiger Chen
Intellectual 750 points
Other Parts Discussed in Thread: BQ76940

hi,

please, whenever i set CHG_ON or DSG_ON, the SCD and OCD turned to 1 immediately that's caused both charge and discharge mosfet failed to turn on.

when set CHG_ON, the SCD turned to 1 immediately

when set DSG_ON, the OCD turned to 1 immediately 

environment : bq76940 connected with 13s without OV or UV errors, without loading connected, 

tiger 

  • 0
    TI__Guru 54715 points

    This is not a normal behavior of the part.

    Look for crosstalk between the CHG and DSG outputs which swing ~12V and the SRP, SRN inputs which respond to 10's of mV signals.

    Also look for ground connections either missing for the IC VSS or SRP/SRN filter, or connection of the SRP/SRN filter capacitors in an unexpected location in the current path.

  • 0 in reply to WM5295
    Intellectual 750 points

    hi thanks for reply, my circuit design is :

    4 pins extended out from (board A) to other board (board B) which is responsible for charge discharge column counter, with the SRP SRN filter, mosfet and its associated components.

    my measure was simple, just measure those 4 pins on board A. the DSG and CHG pin which is directly connect to bq76940's pin 1 and 2, but leave both SRP and SRN floating unconnected, in this test, when set CHG_ON, the SCD turned to 1 immediately, set DSG_ON, the OCD turned to 1 immediately ... 

    also connect board A with board B, and measure those mosfet's gate the CHG signal tried to pull voltage up but immediately down to zero (guess) by SCD reset.

  • 0 in reply to Tiger Chen
    TI__Guru 54715 points

    It sounds like a grounding issue.  Since you do not mention ground, this likely comes to the boards through a mutual connection to battery-.  This will create a loop and as current starts to flow to charge the gate capacitance, the SRN and SRP will start to move.  

    You might try adding a ground between the boards as directly as possible, see if this allows operation for further evaluation. The filter for the SRPand SRN should ideally be very near the the IC.  You might try removing the filter caps on board B and adding them on board A near the IC, be sure to connect the caps to VSS close to the IC.

    However I may misunderstand.  If you observe the reaction with the boards disconnected, it could be crosstalk into the unconnected SRP and SRN.  Again capacitors on board A near the pins may be sufficient, or you may need to short the board sense pins together when board B is disconnected with a similar filter resistance such as ~200 ohm.  It really would be better if the filter resistors were on board A since any noise picked up in the interconnect would be attenuated by the resistors.

  • 0 in reply to WM5295
    Intellectual 750 points
    hi,

    by shorted SRN and SRP with 200 ohm resistor and measure both DSG and CHG pin i have ~12v :)
    then connect both A and B immediately and discovered was mistakenly incorrect connect both (SRN SRP) with (DSG CHG) by upside down the connector plug :(

    problem solved, and will move the SRN and SRP filter cap back to board A for batter noise reduction.

    thank you so much WM5295