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CSD17571Q2: How to drive gate control of load switch with 3.3V signal

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Replies: 3

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Part Number: CSD17571Q2

Is there a simple circuit to drive the gate control of an N-channel NexFET like the CSD17571Q2 when used as a 19V/4A load switch?

The on/off control signal is a 3.3V output from a SN74AVC16T245 buffer. The 19V is the highest voltage on the board.

Or would it be simpler to use a p-channel FET?

Thanks,

Roger

  • Hi Roger,
    Thanks for the questions. If you're using a N-channel FET such as CSD17571Q2 as a high side load switch, you will need to drive the gate to at least 4.5V greater than the input voltage. Since 19V is the highest available voltage on the board, you would need to generate a ~24V bias to adequately drive the FET. This could be accomplished using a boost converter or charge pump. Alternatively, you could use a low side load switch and drive directly from 3.3V output of the buffer. In this case, you would need to select a FET with rds(on) rated at 3VGS.

    A P-channel FET is another option and of course, it is easier to drive. You need to be careful with this approach because you can get current flow thru the device at initial application of the input voltage until the gate charges up to Vin.

    What are your requirements? There are a number of TI load switch, eFuse and hot swap controllers that may work for this application.

    Best Regards,

    John Wallace

    www.ti.com/mosfet

  • In reply to John Wallace1:

    Hi John,

    Thanks for the response.

    After researching the design options some more, I decided it was easier to go with the p-channel option.

    Although I hadn't thought of initial current flow at power up. The switched power rail feeds +12V & -12V regulators. They have thresholds before they even start to power up, so I think it will be OK. I will have to check the power-up sequence and behavior after I build the circuit.

    The general p-channel design that I am looking at going with is shown below.

    I am switching 19V with a 4A max load.

    I couldn't find TI FETs that could handle some of the requirements.

    Thanks,

    Roger Perry

  • In reply to Roger Perry:

    Thanks for the update Roger. Our P-channel devices are fairly limited and I don't see anything that could meet your requirements either.

    Best Regards,

    John Wallace

    www.ti.com/mosfet

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