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Schematic to disable buck from 24V

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  • [Solved] Schematic to disable buck from 24V

    Hi all,

    I would like to disable/enable a buck from a microcontroller (stm32L4) that have I-O 5V tolerant.
    The voltage of enable pin is tied to 24V max --> i can't use a pin from the micro.
    Can you tell me is the design under can be a good choice or do i need to use another transitor between the I/O and EN pin (2 tansistor NPN PNP) ?
    Last edited by Modjo; 03-01-2021, 11:31 PM.

  • #2
    You are totaly wrong about EN pin of this regulator. You have to put a pull down resistor and route it directly to the MCU.

    The EN pin threshold is 1.263V typical. When you make your pins HIGH from MCU side, the output of 3.3V will enable the regulator.

    By the way, you mentioned your MCU pins are 5V tolerant. If you are talking about input,yes this is right. However you are using this pin as output.

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    • #3
      At first thanks for your answer. Why i'm wrong ? see the typical application of TI datasheet under. I can pull the EN pin to VIN --> advantage the buck start without the need of mcu ...
      I just want to know if my schematic can work to pull down the EN pin


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      • #4
        You have to simply pulldown the EN pin and route it to the GPIO of an MCU. Because the threshold of EN pins is 1.263V and the MCU IO voltage 3.3V is enough to enable the regulator.

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        • #5
          Yes i understand and agree with you in the case of it's always the mcu that enable or disable the buck. But in my case (the functionality i need) is to start the buck without the need of an external MCU. This is the raeson i connect enable pin to VIN ! The only reason i need the mcu gpio is to disable the buck when i need it.

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          • #6
            You can use any kind of standard transistor, but using a mosfet is easier. Very often we use 2N7002. If you like, you can find many examples for example in our OpenRex schematic.

            PS: Just be careful what will be the default state of the MCU pin (after power up, very often they are configured as inputs, so you may need an external pull up or pull down resistor) and what should be your PSU (if ON or OFF) at that moment.

            The example below shows a connection of 3V3 output pin to a pin with 5V voltage (that is why the Q13 is used). It is similar to your situation.

            https://www.imx6rex.com/wp-content/u...Production.pdf
            Click image for larger version  Name:	transistor connection.PNG Views:	0 Size:	98.1 KB ID:	16800

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            • #7
              thanks robertferanec this is i would like !
              Just a question, on your schematic you have 5V on Drain pin, if i have for example 24 or 42V we can keep the same mounting or do we need to add a second mosfet like this :

              Click image for larger version

Name:	2021-03-01_21h16_26.png
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              Otherwise i also get some idea on your openRex rex project !

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              • #8
                For Enable pin, you only need one mosfet. The schematic you attached is different - there is one transistor (Q4) used as a switch and the other one (Q9) is used to control the switch (not your situation)

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                • #9
                  Ok thanks for this clarification

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