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  1. #1
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    comparators - pull up or not ?

    hello

    I ordered LMV358IDR comparators to drive FDN340P transistors like on the following schematic

    https://www.arduino.cc/en/uploads/Ma...-schematic.pdf

    but I find it ackward that there is no pullup resistor, I read many times a comparator needed one

    - does it ?
    - since the comparator appears to be powered by 5V USB, if there is no USB plugged in, is it's output low by default, so the FET is open letting any VIN supply go through ? how can I be sure of this ?

    regards
    Last edited by rompelstilchen; 8th November 2018 at 17:01.

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    Re: comparators - pull up or not ?

    LMV358 are rail-to-rail OPs, not comparators, but can be of course used as comparators as well. The output is push-pull, no pull-ups required or useful.

    The OP can work for the application, but I wonder why you don't use single channel LMV321?



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    Re: comparators - pull up or not ?

    Hi,

    Most comparators have open_collector or open_drain output, but there are also push-pull output comparators.
    LMV358 is no comparator, it is an OPAMP. OPAMPs usually have push-pull ouput.

    An OPAMP is not the same as a comparator. Select the proper device for your application:
    see this: https://www.edaboard.com/showthread....=1#post1619298

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: comparators - pull up or not ?

    actualy I have 2 usb connectors, one for a teensy , one for an arduino

    they dont have any USB power switching

    and indeed I should have ordered 321

    can you answer my 2nd question too?

    appreciate your help

    - - - Updated - - -

    it's the official arduino schematic, so I'll reproduce it

    but I was wondering what about no USB power -> no power for the comparator -> what is the state of the FET transistor ?



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    Re: comparators - pull up or not ?

    The comparator is powered by +5V node, derived from Vin and Vusb. In so far I don't understand the question.

    You should pay attention to the MOSFET direction. It's main function is block reverse feeding the USB from VIN. It switches off if Vin > 6.6V, not strictly the expected safe behavior of an USB client according to USB standard.



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  6. #6
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    Re: comparators - pull up or not ?

    if USB (through mosfet) and VIN (through VREG) both produce 5V and reach the same 5V point in the schematic

    then a simple diode would do the trick, right ?



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