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Original question:

TPS56121: tps56121

TPS56121: TPS56121 EN pin start up at low temperature

Will Wang47
Expert 5510 points
Part Number: TPS56121

Hi team,

Customer is using TPS56121 in their project, and their schematic as follow. Now they meet one question during low temperature start up.

They find in their circuit, the start up time take more time, and they test the EN/SS pin voltage, find during low temperature, it takes more time to let it goes to normal, please see the following EN test waveform, Could you help check why this happen? Does there any issue with the schematic?

25C:

-5C:

-15C

-40C:

Best Regards,

Will

  • 0
    TI__Genius 16760 points

    The 4 cycles of soft start, and then output rising in -15C is similar to the OC triggering.   Can you share a pcb layout with me?    Try increasing the soft start time and increasing the Rtrip value to check if the behavior is reduced

  • 0 in reply to David_Daniels
    TI__Expert 5510 points

    Hi David,

    I'll send you a mail for a more clear view of the PCB layout. BTW, could you help comment the Rtrip value? Which resistor does this resistor is in customer schematic?

  • 0 in reply to Will Wang47
    TI__Genius 16760 points

    R98 connected to the ILIM pin sets the current limit.    Add a capacitor to the VDD to GND on top side of board and check if operation improves

  • 0 in reply to David_Daniels
    Prodigy 10 points

    Hi David,

    Thanks for your guide.

    I've a question about the design of the soft start capacitor and the Rtrip,  does it just impact the start time, or will lead to failure of the output?

  • 0 in reply to Guisheng Li
    TI__Genius 16760 points

    Section 7.3.2 of datasheet link describes the start up sequence with EN/SS.   Section 7.3.2.2  and 7.3.3 describe the OCP and soft start time.   

    The soft start time is adjusted to minimize inrush current during start up.   A short soft start time may have higher peak currents during start up.

    The Rtrip sets the OC level.  With a fast soft start and a lower OC level, it is possible to trip the OC level. 

    But it is most critical for the device to have a stable input voltage supply on VDD and BP, a capacitor closer to the pin will help. The BP voltage is the supply to the internal circuits.