Author Topic: Circuit for powering a "dumb" USB device?  (Read 842 times)

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Offline ryanmillsTopic starter

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Circuit for powering a "dumb" USB device?
« on: October 30, 2019, 07:05:18 pm »
I only have a basic understand of USB. Redesigning a PCB so I can power an LCD screen (https://www.adafruit.com/product/2232) directly off the board. I assume the LCD is a "dumb" device in that there is no need for data pins to negotiate current like you might have with a phone. The board already has a stable +5 volt rail with plenty of amps to spare courtesy of a 5v LDO regulator (LM1084ISX-5.0/NOPB: http://www.ti.com/lit/ds/symlink/lm1084.pdf).

I assume I can just run that +5v rail and maybe a bypass cap into a standard Type A header and call it a day? Or are there some standard protections I should be adding?
 

Offline magic

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Re: Circuit for powering a "dumb" USB device?
« Reply #1 on: October 30, 2019, 07:55:47 pm »
Most USB ports use a polyfuse to prevent 5V rail collapse and/or regulator overload in the event of external short circuit.
Whether you anticipate external shorts to happen and if/how your device is expected to survive them is another thing.

If you want to be pedantically compliant with USB spec, you should be able to withstand indefinite short between any pair of USB pins.
 
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Offline ryanmillsTopic starter

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Re: Circuit for powering a "dumb" USB device?
« Reply #2 on: October 30, 2019, 09:20:57 pm »
Most USB ports use a polyfuse to prevent 5V rail collapse and/or regulator overload in the event of external short circuit.
Whether you anticipate external shorts to happen and if/how your device is expected to survive them is another thing.

If you want to be pedantically compliant with USB spec, you should be able to withstand indefinite short between any pair of USB pins.

Thank you, is there a reason you might pick a Poly over a Ceramic in this application?
 

Offline Brumby

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Re: Circuit for powering a "dumb" USB device?
« Reply #3 on: October 31, 2019, 04:21:14 am »
A polyfuse resets itself after it has cooled down sufficiently.
 


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