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Thread: Direct drive MOSFET (DMG3406L) from Teensy digital pin?

  1. #1
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    Direct drive MOSFET (DMG3406L) from Teensy digital pin?

    I need the Teensy to switch power on/off for a few system components drawing less than 200mA @ 3.3V total. Can I drive a MOSFET like the DMG3406L directly from a Teensy digital pin? I am thinking it will need a series resistor to limit current but not sure... e.g. something like:

    Click image for larger version. 

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  3. #3
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    Quote Originally Posted by ct1 View Post
    I need the Teensy to switch power on/off for a few system components drawing less than 200mA @ 3.3V total. Can I drive a MOSFET like the DMG3406L directly from a Teensy digital pin? I am thinking it will need a series resistor to limit current but not sure.
    The DMG3406L is only rated for 4.5V gate drive, but will probably work at 3.3V (for these low currents).
    You need to use a switching configuration when switching, ie a common-source circuit with the load on the drain
    of the MOSFET. For an n-channel FET this means using low-side switching, for a p-channel FET use high side
    switching.

    Your circuit is a source-follower which will not work, perhaps only giving 1V or so to the load.

    Yes its wise to limit the pin current, FET gates look very like capacitors and will otherwise pull high spikes of
    current.

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    Ah, I had the wrong MOSFET specified, for this high side switch the circuit would be:

    Click image for larger version. 

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    As long as the digital HIGH and the Vdd supply voltage are the same (3.3V) this should be OK as described in BaldEngineer article, right?

    Also I am wondering what happens when the Teensy goes into deep sleep or hibernate, do the digital pins float? What will that do to this switch? I need the power to stay OFF when the Teensy is in deep sleep/hibernate.

    Thanks for the help!

  5. #5
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    Quote Originally Posted by ct1 View Post
    Ah, I had the wrong MOSFET specified, for this high side switch the circuit would be:

    Click image for larger version. 

Name:	mosfet.png 
Views:	12 
Size:	22.4 KB 
ID:	25976

    As long as the digital HIGH and the Vdd supply voltage are the same (3.3V) this should be OK as described in BaldEngineer article, right?

    Also I am wondering what happens when the Teensy goes into deep sleep or hibernate, do the digital pins float? What will that do to this switch? I need the power to stay OFF when the Teensy is in deep sleep/hibernate.

    Thanks for the help!
    A resistor in the gate line also will help suppress oscillations that could occur when the device is switching. Regarding the sleep/hibernate question, if the digital pin becomes a high impedance you can always put a relatively high-value resistor between 3.3V and the digital pin. 10K or probably even 100K would suffice.

    The pullup wouldn't be a bad idea even if the output remains at its programmed level/impedance -- if the transistor connections go through a connector. Losing the gate-drive connection due to a connector problem (or if the wire breaks) might be problematic, so the pullup resistor would be kind of a fail-safe. Of course, this means that you want to install the pullup resistor on the transistor side of the connector.

    Edit: I might go so far as to put an ESD protection diode between the gate and source as well. Does my background in Reliability show

    Mark

  6. #6
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    More reading on the Teensy tells me a digital output pin will retain its state (voltage/current) in sleep state, so that is good.

    What is bad is that now I want to build it... and DMG3415 devices (and similar) are nowhere to be found due to the semicon shortage.

    While poking around I found the OnSemi FPF2123-FPF2125 devices which are specifically designed to be driven by digital logic and have some additional features like over current/short circuit protection, thermal shutdown, etc.

    https://rocelec.widen.net/view/pdf/v...03590112-1.pdf

    Those are available at the usual places and seem to be as simple to use in a design as the MOSFET but with some protection features.

  7. #7
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    Why not use a regulator with an ENABLE input to control the voltage instead. Something like TLV70233 or TPS7A6133. Feed from a low voltage such as 5.0v to minimize power dissipation. There are also small adapter PCBs available to make them breadboard compatible.

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