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LP38502-ADJ: Application suggestion for driving Enable pin

Greg Reynen
Prodigy 20 points
Part Number: LP38502-ADJ

Hi,

I'm trying to drive the enable pin of an LP38502 directly from part number APX809-29SAG-7 and I find that the supervisor circuit is unable to assert a new logic level (stays low forever after powerup) though it seems to be able to hold the enable high if I force the net high by shorting it temporarily to a 3.3V rail (short pins 1 and 2 temporarily). The datasheet for the LP38502 seems to say it only sinks something around 1 nA which the supervisory circuit can surely provide. I've tried using a 10 kohm resistor between 3.3V and the enable pin (pins 1 and 2), I've tried no resistor between them, and I've tried various values between 100 ohms and 10 kohm to no effect.

If i lift the enable pin, the supervisor chips output does get pulled high after the expected timeout. It is only when connected to the LP38502 that it is unable to assert.

Any other tips on utilizing the enable pin would be great.

The schematic is below. Note that LP38502 chips are populated, not the LP38500 which I know internally pull the enable pin high.

  • 0
    TI__Genius 13375 points
    Hi Greg,

    The scenario you describe is very bizarre, that supervisor should be able to drive the enable pin of the LP38502 since it is just the gate of a mosfet and therefore requires very little current. Unfortunately your schematic didn't come through can you try reposting it? Currently the only thing I can think of is that perhaps the board has some unintended leakage path from the enable pin to ground, solder flux or something else, which is too much for the supervisor to overcome.
  • 0 in reply to Kyle Van Renterghem
    Prodigy 20 points
    Thank you for the reply.

    The cause ended up being very short duration downward spikes on my 3.3 V. The supervisor was then staying in reset as a result - exactly as it is meant to do. Not noticing that first was my biggest mistake!