Electronics > Projects, Designs, and Technical Stuff

USB memory stick voltage

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Simon:
To my knowledge USB is 5V +/-5% = 4.75 ta 5.25V. Is it reasonable to expect a memory stick to work off a lithium battery directly at 3.7-4.2V ?

magic:
Dropout voltage of the built-in regulator which provides 3.3V for USB comms may be an issue.
As for flash itself, I think it runs at 3.3V or less too. SD cards don't even have a 5V rail at all. So again, a question of whether the LDO will deliver.

Simon:
but if the device is powered directly off the battery this will vary from around 4V fully charged to lower voltages. So would it be reasonable to expect a memory chip that has the built in USB interface to work?

ogden:

--- Quote from: Simon on February 26, 2019, 08:39:45 am ---but if the device is powered directly off the battery this will vary from around 4V fully charged to lower voltages. So would it be reasonable to expect a memory chip that has the built in USB interface to work?

--- End quote ---

Typical 3.3V controllers most likely have +/- 10% VCC range (ST72681 datasheet states 3.0V as absolute minimum). Typical LDO dropout is 0.5V or so. For hobby/DIY I would set "battery empty" voltage little higher and state "there still may be some flash sticks that does not work". For commercial product - way too much risk. Could be so that some very popular flash drive have 1v drop regulator with no bulk capacitor at all. All this "IMHO" w/o actual experience of testing flash drive brownout voltage levels, obviously

Gyro:

--- Quote from: Simon on February 26, 2019, 08:39:45 am ---but if the device is powered directly off the battery this will vary from around 4V fully charged to lower voltages. So would it be reasonable to expect a memory chip that has the built in USB interface to work?

--- End quote ---

It depends. If it's a one-off for yourself, then try it. If it's any sort of quantity for someone else (customer) then you're using the device outside its specified operating range,

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