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Original question:

LM51501-Q1: LM5150 fast turn off bypass FET with lowest power consumption

LM51501-Q1: LM5150 Gate drive question for dual source

Shaquille Chen
Intellectual 1770 points
Part Number: LM51501-Q1
Other Parts Discussed in Thread: LM5150RUMEVM

Dear Team,

We put two of LM5150 EVM board for dual source input.

The block diagram as below:

We found that the bypass FET on LM5150 #2 EVM board didn't turn on when #2 board input disconnect.

Why the VSG higher than Vth but didn't turn Q1 on?

The test procedure as below:

1.Set VIN of LM5150 #1 =3V and Load current of Dummy load is 0.3A
2.Set VIN of LM5150#2=12V and power on

3.   After VOUT is stable and change  LM5150#2 power to 0V

4. Check the waveform of measurement point 1,2,3,4 when VIN of LM5150 #2 are changed

Thanks

Shaq.

  • 0
    TI__Mastermind 21720 points
    Hi Shaquille,

    Is the bypass MOSFET a p-channel device? If that is the case the gate voltage should be pulled low to enable the switch.

    Also please note that in SS mode operation there needs to be a voltage on the input to make the device functional.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    TI__Intellectual 1770 points
    Hi Garrett,

    Yes, it is PFET. We use two "LM5150RUMEVM" for this experiment.
    Please refer to EVM guide page 9.

    The gate voltage of Q1 is 3.6V as channel 2.
    The source voltage of Q1 is 6.8V as channel 4.

    Since this is PFET, the VSG > Vth should force PFET on but it didn't.
    We want to know the reason why.

    Thanks
    Shaq