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Hot!MCP7940M MFP open drain output max VDS

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edprog
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2019/04/26 16:28:34 (permalink)
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MCP7940M MFP open drain output max VDS

Hi
 
I am using the MCP7940M and I have the MFP output, which is open drain, pulled up to a rail higher than VDD. The datasheet says that no input or output can be more than VDD +1V, but this is an open drain FET output. My question is then, if this was a normal FET the max output you could connect to this output would be governed by the VDS of the FET inside the MCP7940M. I have asked Microchip support but they just quote back to me absoluter ratings on datasheet i.e. VDD +1V. Any help would be appreciated,
 
EB

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    qhb
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    Re: MCP7940M MFP open drain output max VDS 2019/04/26 17:42:58 (permalink)
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    edprog
    ...
    The datasheet says that no input or output can be more than VDD +1V, but this is an open drain FET output.

    If the specs do NOT say "except for MFP", then that is what you have to work with.
    You can not make assumptions like "it is just an open drain FET". For all you know, there could be a protection diode to VCC.
     

    I have asked Microchip support but they just quote back to me absoluter ratings on datasheet i.e. VDD +1V.

    In this case, this is the only answer they could give.
    If you choose to ignore it, then you have no come back if you damage chips.

    Nearly there...
    Mysil
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    Re: MCP7940M MFP open drain output max VDS 2019/04/26 18:22:40 (permalink)
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    Hi  edprog,
    The MFP output transistor is part of the same silicon chip as the rest of the device,
    it is Not a separate transistor that is isolated from the device substrate.
    Thus you cannot use reasoning that is valid for a discrete FET transistor in a open drain configuration.
    There is some trick applied in the chip design that allow some output pins to tolerate open drain voltage slightly higher than device power supply Vcc, 
    but only within limitations stated in the datasheet.
     
        Mysil
    edprog
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    Re: MCP7940M MFP open drain output max VDS 2019/04/27 05:27:52 (permalink)
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    Ok was just trying to get away without using an external FET but looks like I will have to use one. Thanks for your advice!
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