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LP3985 3v3 on start up the output follow the 5v input to 4.6v for 50mS then settle to 3v3 this destroys all 3v3 devices on the power rail???

Other Parts Discussed in Thread: LP3985, LM46002

total 6x 10uFcermic  caps on out puts and Apx 20 100nF ceramic caps  max load 130mA

LP3985 over volt problem.docx

  • Hello Stewart,

    Where is the 100uF capacitor on the input to the LP3985 located? How far away is it? Also what type of capacitor is it because it looks like an electrolytic. First thought would be to replace the input cap C107 with a 1uF ceramic.

    The next question is can you provide me with a little bit more information on the output capacitors? What happens if you change C109 or whatever capacitor is closest to the output to a 1uF X7R ceramic cap?

    One other thought is that the LDO is coming on line before the the LM46002 has stabilized. The part is not guaranteed to operate properly when the input voltage is not within the desired range (7.4.1). The rise-time of the buck converters output might be confusing the LP3985. I had hoped to recommend using the PGOOD pin of the LM46002 but the polarity is active low which is the opposite of what the LP3985 needs.
    My thought would be to place an RC circuit connecting the 5V rail of the EN pin. This will allow the 5V rail to stabilize for the the LP3985 tries to start up. This can be tested by cutting the trace to the EN pin an placing this circuit in-line. You should select the cap to be small and then design the time constant so that the EN pin does not come up past about a 1V until after the 5V rail has stabilized.
  • There is nothing at all in the LP3985 datasheet 'Start-up Time' graphs that suggest any normal LP3985 device would behave like this.

    I would suggest removing the bypass capacitor and see if anything changes. My first, and only, guess is that the bypass cap off pin 4 on the SOT-23 package is somehow connected to either Vin or Vout, rather than Ground. This would pull the bypass pin high as the Vin or Vout voltage rises and cause Vout to go high. Once Vin or Vout stops moving the bypass cap would eventually discharge and the bypass pin voltage, and Vout, would return to normal.