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comparators - pull up or not ?

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rompelstilchen

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hello

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

https://www.arduino.cc/en/uploads/Main/arduino-uno-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
 
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FvM

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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?
 

KlausST

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rompelstilchen

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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 ?
 

FvM

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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.
 

rompelstilchen

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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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