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LM25118: LM25118 can't start up at 3.7V input to 9V output

Eric Chen4
Expert 2370 points
Part Number: LM25118


Hi,

Now I use LM25118 to design 3.7V~20V input to 9V/1A output schematic and use TI Webench tool to do simuation as below.

But I find can't start up when 3.7V input and UVLO is 13mV  --> Why?

Then remove Enable pull-up R and can find UVLO is normal.  --> Why?

Please kindly provide some suggestion for our check again.

Thanks.

20181016-webench_design_5834391_10_285013390.pdf

Regards,

Eric

  • 0
    TI__Mastermind 21720 points
    Hi Eric,

    The LM25118 requires at least 5V to start-up. With the VCCX pin connected to the output voltage it can operate down to 3V on the input.

    Please double check the connections on the UVLO pin. I cannot think of any reason that the UVLO pin would be pulled o 13mV.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Expert 2370 points

    Hi Garrett:

    Yes, I already double confirm UVLO stage when input voltage is 3.7V.

    But Remove Enable pin pull up resistor then find UVLO pin voltage is 1.5V.

    May I know what relation between Enable and UVLO pin?

    Please kindly provide some suggestion for our check again.

    Thanks.

    Regards,

    Eric

  • 0 in reply to Eric Chen4
    Expert 2370 points

    Hi Garrett:

    I check LM25118 spec that find VIN>5V to start the controller.

    Please kindly check if have risk when VIN<5V and can't match TI Webench Tool result as attached.

    Thanks.

    4370.20181016-webench_design_5834391_10_285013390.pdf

    Regards,

    Eric

  • 0 in reply to Eric Chen4
    TI__Mastermind 21720 points
    Hi Eric,

    The LM25118 requires 5V to startup correctly. If VIN is below 5V the device might not startup. When the VCCX pin is connected to the output or and auxiliary rail the LM25118 can operate down to 3V after start-up has occurred. Regardless of the setup the VIN needs to be 5V to startup. If the device needs to start-up below 5V the VCC rail should be externally biased above the VCC UV threshold.

    Enable control if the device is in normal operation or shutdown mode. UVLO controls if the device is in standby mode or normal operation. For switching to occur the UVLO must be above ~1.2V and the EN pin must be above 3V.

    Thanks,

    Garrett