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TPS40422: Power supply with TPS40422 short currnet test issue

HC Chen
Prodigy 80 points
Part Number: TPS40422

Hi

 

PS: we set the per phase current limit 22A, so total current limit is 44A.

  1. Why the Short Then On test the current is so match high(85A) than OCP and On Then Short test?

      

  2. Why the negative voltage spike of -1.4V for ~60us, does it have any sulotion?

.

  • 0
    TI__Mastermind 24075 points

    HI HC, 

    First, can you check if IOUT_CAL_GAIN is equal to inductor DCR value?

    The OCP is for inductor peak current, not load current. When output is shorted, the load current could be much higher than inductor current because output cap discharging current could be large. 

    For the negative voltage spike, it could be caused by cable inductance. Can you short output with a short wire/cable close to VR side?

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Prodigy 80 points

    Hi Qian

    1. Yes, we seted the IOUT_CAL_GAIN is equal to inductor DCR value.

    2. We short the long of wire, and close to the voltage output side, the negative voltage spike is smaller, but -800mV. Anyelse to improve it?

     

  • 0 in reply to HC Chen
    TI__Mastermind 24075 points

    Hi HC, 

    Can provide a picture showing how output is shorted and where Vout is measured?

    Also, can you use scope probe to capture Vout, SW1 and SW2 waveform at the same time? Just want to make sure the undershoot is caused by external setup, not TPS40422. 

    Thanks

    Qian

  • 0 in reply to Qian Chen
    Prodigy 80 points

    Hi Qian

    We use switch to do the short test.

    Result:

  • 0 in reply to HC Chen
    TI__Mastermind 24075 points

    Hi HC, 

    The waveform shows the switch node stops switching when Vout start to drop, it indicates the negative Vout is not related to TPS40422. 

    In your test setup, the short point is at inductor side, the measurement and Cout is at load side.

    When the mechanical switch is turned on, the output cap will be discharged like below. 

    After Cout is discharged to 0V, the stray inductance between Cout and switch will continue discharge Cout because the current in stray inductance can not stop suddenly. That's how negative Vout comes. 

    Can you short output at load side, close to Cout?