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TPS54560B-Q1: Output waveform abnormal

Wayne Liu
Expert 8605 points
Part Number: TPS54560B-Q1
Other Parts Discussed in Thread: TPS54560

Hi

When customer disconnects the input power supply, the output voltage waveform is abnormal shown as picture 1, EN is as picture 2, schematic is as picture 3. I suspected the falling edge of EN is too slow, so removed C418, but it didn't work. Could you please help tell me why? Thanks. 

  • 0
    TI__Mastermind 27045 points

    Hi Wayne,

    The EN pull up resistor R199 is NC, right?

    I think the restart issue is caused by VIN UVLO of TPS54560:

    • when Vin drop below 4V, TPS54560 stop switching
    • Then input voltage will increase as the input current drop to zero
    • If Vin goes back to 4.3V or higher, circuit will re-start and charge up output again.
    • this behavior will repeat till input voltage can go back to 4.3V anymore.

    customer can design EN divider to program system UVLO as described on data sheet, for example, start voltage at 5V, stop voltage at 4V

    B R

    Andy

     

  • 0
    TI__Expert 8605 points

    Andy,

    Thanks for your quick feedback.

    The resistor for EN is not NC, it is placed.

    I don’t understand your item 2, why the input voltage will increase after drops below 4V? This abnormal waveform happens when we disconnect the input power supply, so the input voltage should not increase after drops below 4V.

  • 0 in reply to Wayne Liu
    TI__Mastermind 27045 points

    Hi Wayne,

    As we talked on the phone, the input capacitors have ESR and wiring resistor, so the input voltage will increase if the input current decreased.

    B R

    Andy