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Original question:

LM66100: LM66100 is not working as expected

LM66100: Valid PFET-driving topology?

David Keck
Prodigy 30 points
Part Number: LM66100

Hello,

I'm trying to implement this logic with LM66100 + PFET:

  • 5V present: VOUT = 1V8
  • 5V absent: VOUT = 3V

This is my schematic:

On one hand, it seems this isn't a valid topology because: (1) the simulation voltages (shown in the schematic) suggest that the LM66100 drives VOUT despite CE>VIN, and (2) I'm guessing the body diode of the LM66100's internal FET will conduct when VIN > VOUT.

On the other hand, the LM66100 simulation model is apparently buggy (according to this and this), so perhaps I'm hitting a bug.

Is this a valid topology for the LM66100? 

Thanks,
David

  • +1
    TI__Guru 57737 points

    Hi David,

    Thanks for reaching out to us.

    When CE/ pin voltage is greater than IN pin voltage, the internal FET gate is pulled low. but as you rightly said, the body diode of the FET still conducts when IN>OUT. 

    In the image shared by you, IN = 3V, CE/ = 5V and hence you can see that,

    1. The internal FET is disabled which is indicated by ST pin voltage = 2.2 mV which means low.
    2. OUT = 2.21 V which is (IN- Diode Drop). 

    This is an expected behavior of the IC. Do let me know if I am missing anything here. 

    Looks like you are trying to realize ORing with a discrete MOSFET as shown in the below circuit of the datasheet. You can refer to datasheet section '9.2.3 ORing with Discrete MOSFET' for test results of this circuit. 

  • 0 in reply to Praveen GD
    Prodigy 30 points

    Thanks very much for the confirmation Praveen.

    Indeed I'm trying to OR between 3V and 1.8V, similar to the datasheet circuit, but the critical difference is that a third 5V signal needs to be the trigger to switch between them.

    I'm coming to the conclusion that LM66100 isn't well suited for this. Ideally I'd use a power mux, but I haven't found any with low enough power consumption for my purposes, hence experimenting with LM66100.

    Anyway, it looks like I can solve this problem using ~4 discrete FETs, so I'll experiment with that...

    Thanks again,

    David

  • 0 in reply to David Keck
    TI__Guru 57737 points

    Hi David,

    Do let us know if you have any further questions. I am closing this thread for now.