USB memory stick voltage - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 08:15:41 am
content: 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 ?

author: magic
author_rank: Super Contributor
timestamp: February 26, 2019, 08:32:56 am
content: 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.

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 08:39:45 am
content: 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?

author: ogden
author_rank: Super Contributor
timestamp: February 26, 2019, 09:39:41 am
content: Quote from: Simon on February 26, 2019, 08:39:45 amTypical 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

author: Gyro
author_rank: Super Contributor
timestamp: February 26, 2019, 10:04:26 am
content: Quote from: Simon on February 26, 2019, 08:39:45 amIt 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,

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 10:13:12 am
content: I am trying to work out why a charity i volunteer for has USB memory stick MP3 players that fail to recognise an off the shelf memory stick. Older players have a 3.9V output on the USB socket which I think is straight from the battery. I have just checked a newer one and it was 4.8V so I suspect they have a DC/DC converter in it.

author: ogden
author_rank: Super Contributor
timestamp: February 26, 2019, 12:09:33 pm
content: Quote from: Simon on February 26, 2019, 10:13:12 amOne particular stick or any kind you try? Player with 4.8V output works well, right?Quote3.9V is unloaded voltage, right? I would look at USB socket voltage during operation using scope as well - to check for voltage dips.

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 12:43:44 pm
content: The voltages are unloaded. I actually had a problem with the type of player producing 4.8V first but it may not have been fully charged so even a DC/DC may have struggled. Yes ideally I need to look at the loaded down rail but that is easier said than done.

author: ogden
author_rank: Super Contributor
timestamp: February 26, 2019, 01:05:44 pm
content: Quote from: Simon on February 26, 2019, 12:43:44 pmProper fault analysis requires to see what's inside that piece of [****] anyway. Two cents: Find some old usb flash like kingston datatraveller - those are easy to open, measure on flash side.Other option - open something like this:https://cdn-shop.adafruit.com/970x728/1852-03.jpg. At least get some for next USB voltage struggle - very handy.

author: magic
author_rank: Super Contributor
timestamp: February 26, 2019, 05:16:57 pm
content: USB type A male and female connectors for PCB THT mounting are easily available and come in 2.54mm terminal pitch or something close enough to that. Mount two on opposite sides of a tiny perfboard, connect them the obvious way and probe to your heart's content. Don't worry about signal integrity too much, similar things are done in computer front panel ports.Pro tip: make the 5V connection through a jumper so you can disconnect it and apply proper 5V power to the flashdrive. I think this will answer your questions.Pro tip 2: I don't have the details figured out yet, but I suspect a properly sized capacitor between D+/D- would inhibit the High Speed handshake while still allowing Full Speed to work. This could be handy for sniffing the bus with a logic analyzer if it's a logic problem.

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 05:40:48 pm
content: well i will go back to the supplier and see what they say. ultimately I can't fix stuff here and the under voltage units are out of warranty.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 26, 2019, 06:06:09 pm
content: Quote from: Simon on February 26, 2019, 08:39:45 amWould it be reasonable to expect any device to work out of spec?Whereas we can reasonably expect all of these to be internally powered at 3.3V, given their size and cost, it's very likely they use a linear regulator. Given again the cost factor, it's also very likely they include lousy LDOs that are not that low drop-out at all. A very common "LDO" that we can see in many cheap devices is the ubiquitous AMS1117 which is dead cheap, for instance. It has over 1V of drop-out voltage, so given that for most of the useful life of a LiPo battery, the voltage is around 3.7V-3.8V, you'd end up with 2.7V or less. So not only may some ICs misbehave, but one of the most problematic point I see would be a significant mismatch in the logic 'high' voltage between the host and the memory stick, potentially causing problems with the USB data lines.The power supply stage of those memory sticks is likely to be pretty different from one device to another.Powering USB devices directly from a LiPo battery looks like a pretty lousy design here IMO.

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 06:14:22 pm
content: i really don't know why anyone thinks there are any voltage regulators in here. The 3.9V has not come through a "LDO" or it would be hardly 3V. I suspect the circuitry is 3.3-5V tolerant but the memory stick maybe not.

author: ogden
author_rank: Super Contributor
timestamp: February 26, 2019, 06:17:13 pm
content: Quote from: Simon on February 26, 2019, 05:40:48 pmManufacturer shall fix problems anyway, thou make sure that your tests confirm/demonstrate voltage problem and other potential controller/disk_size/file_system/file_naming software/compatibility problems are excluded from suspicions.Quote from: Simon on February 26, 2019, 06:14:22 pmFlash sticks receive 5V from USB but controllers are 3.3V+1.8V IC's. Indeed there is some regulator, in the flash drive - separate or IC-integrated.

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 06:29:09 pm
content: So you think there might be a regulator in the memory stick that drops the voltage to 3.3V which may be hard to achieve with 3.9V in the first place?

author: ogden
author_rank: Super Contributor
timestamp: February 26, 2019, 06:39:51 pm
content: Quote from: Simon on February 26, 2019, 06:29:09 pmI don't think in a sense "guess". I know.https://eu.mouser.com/applications/usb-flash-drives-myths-dispelled/

author: Simon
author_rank: Unknown
timestamp: February 26, 2019, 07:27:27 pm
content: I see, that could be the problem.