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WEBENCH® Tools/LM25118-Q1: UVLO pulled to 0.9V when first being powered up

Yantao Dong
Intellectual 256 points
Part Number: LM25118-Q1
Other Parts Discussed in Thread: LM25118

Tool/software: WEBENCH® Design Tools

We are using LM15118Q1 for first stage regulator and generate 13.5V for the system, and there is another separated LDO supplies 3.3V for a MCU .the MCU detectes the external switched voltage through a detect IC (ACC detection),  when powering on the system, the MCU can detect a H level and then drives the "SEPIC-ON"  which is conneted to UVLO, and also UVLO has an external 15Kohm pull down resister to ensure  the system to be shut down when there is no ACC.

THE ISSUE  is  when the first time powered up , sometimes the UVLO is pulled to 0.9V instead of 3.3V, and there is no output . ACC off and then turn ACC on  can make the buck boost work , which means turn off UVLO(SPEIC-ON) for a while and then turn on.

We also change the UVLO pull down resister larger to 1Mohm ,which can make it better but still fail in high or low temperature test.

Any one and help me to explain what's the reason about it.

Thanks 

Yantao

 

  • 0
    Intellectual 256 points

    we also changed the delay time from ACC ON detection to SEPIC on，

    no delay  ：  easy to reproduce the issue

    delay 20ms：easy to reproduce the issue

    delay 1000ms：issue does't happen yet

  • 0 in reply to Yantao Dong
    TI__Guru* 78440 points

    Hi Yantao,

    Thank you for  using the LM25118. According to the datasheet you will have to establish 1.271V at the UVLO  pin, and 3V at the VIN pin, to guarantee a successful turn-on.  If you just see 0.9V at the UVLO, it means your external circuit does not establish the correct turn-on threshold voltage at the UVLO, and at the EN pin of your schematic.  Check if your MCU's GPIO can drive the UVLO and EN pin.  If not, you may insert a buffer in between.

    Thanks,

    Youhao Xi, Applications Engineering