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Mosfet to turn off power source

khieu sy
Prodigy 80 points
Other Parts Discussed in Thread: CSD25310Q2

I have an issue related to mosfet

Like an image bellow i wanna use Pmosfet to cutoff power but when i put high logic to G pin -> I got 2.2-2.4 voltage at Pin3 of mosfet

I don't know why.

do anyone have any idea about this issue?

Thanks,

  • 0
    TI__Mastermind 32650 points

    Hi Khieu,

    This forum is for technical questions on TI Power Switch devices. 

    For more information on TI Power Switch devices, please visit this link: https://ti.com/powerswitches 

    For 3.3V systems, I would recommend looking at our Load Switch portfolio.

    Thank you for your interest in TI power Switches.

    Shreyas

  • 0
    TI__Mastermind 39380 points

    Hello Khieu,

    Thanks for the inquiry. The gate of the P-channel FET needs to be pulled up to 3.3V to turn it off. I looked at the datasheet for the PFET shown in the schematic and it specs a minimum threshold voltage of -0.3V for -1mA of drain current at VDS = -3V. It's possible that the logic signal is not pulling up high enough to fully turn off the PFET. In order to develop 2.2V across the 100kΩ resistor, R33, the current thru the FET needs to be about 22μA. I would recommend measuring VGS to make sure it is less than 0.3V. Also, keep in mind that the threshold voltage has a negative temperature coefficient and the magnitude decreases with increasing temperature. The drain-source leakage current has a positive temperature coefficient and increases with temperature. Lastly, at power on, depending on how fast the 3.3V ramps up at the source of the FET, there can be some current flow thru the FET until the gate charges up to the source voltage. Are you looking for FET recommendations to replace the device you are currently using? If so, I would recommend the CSD25310Q2 from TI.

    Best Regards,

    John Wallace

    TI FET Applications

  • 0 in reply to John Wallace1
    Prodigy 80 points

    Hi 

    Thanks for your ideal

    I measured Vgs (pin 1 and 2) and got 0.03-0.05 V but still have 1.2V on VCC_3V3 ( temperature  of my LAB is 23 ° C) No Load connected at VCC_3V3

    Pin 1 Just connect with C48 and R32.

    CSD25310Q2 -> Let me try with your Ideal but i don' think the problem related to the Mosfet.

    Thanks

  • 0 in reply to khieu sy
    TI__Mastermind 39380 points

    Hi Khieu,

    Is there any other path for current to flow to VCC_3V3? Perhaps whatever is powered from the on VCC_3V3 is also powered from another rail in the system? I have seen applications where the powered device is connected multiple voltage rails and there is leakage between the two rails. With VGS between 0.03V and 0.05V, there should be very little leakage thru the P-channel FET.

    Thanks,

    John

  • 0 in reply to John Wallace1
    Prodigy 80 points

    Hi John sorry for my late

    Don't have any device connected with VCC_3V3

    I know Mosfet actualy is P and N and will have leakage current but it will very small when Mosfet is turn off but >1V is too big.

    Thanks

  • 0 in reply to khieu sy
    TI__Mastermind 39380 points

    Hi Khieu,

    I'm not sure why you are getting 1V at the output when the MOSFET is turned off. Have you tried shorting the gate directly to the source? Have you contacted the MOSFET vendor to get their assistance.

    Thanks,

    John

  • 0 in reply to John Wallace1
    TI__Mastermind 39380 points

    Hi Khieu,

    Since I have not gotten a response from you, I assume your issue has been resolved and will close this thread.

    Thanks,

    John