USB current is by limit or are absolute maximum ratings

Thread Starter

muhzd

Joined May 25, 2009
34
Hi guys,

While designing USB powered device, I thought maybe I should pay care if the user plugs it to the desktop PC's USB port or laptop.
My device draws more than 500mA that the motor will have to run longer to finish the job. But the question is is asking more current from the desktop PC's USB port is damaging or simply the current is not allowed to?
Thanks.
 

dl324

Joined Mar 30, 2015
16,845
According to the USB spec, all devices must negotiate with the host before they can draw more than one USB load. For USB2, that's 100mA. For USB3, it's 150mA.

Max allowed current from a USB2 host is 500mA, for USB3, it's 900mA.

The spec also states that a USB host is to deal with overloads with a self resetting fuse, or other self resetting manner. PTC fuses can fail. Behavior of a non-compliant host is unknown.
 

Papabravo

Joined Feb 24, 2006
21,159
What dl324 is trying to say is that you are running way to close to the edge. USB cables are not meant to deliver power to motors and running right up to the limit has serious implications for degrading the user experience. Rethink your strategy -- it will result in superior performance and customer satisfaction.
 

Thread Starter

muhzd

Joined May 25, 2009
34
USB power supply support was one requirement. It must be a mobility issue and powerbanks was very synonymous. As for technical development with eyes on the user consumer I asked what if the device was plugged into the USB port of a desktop computer or laptop. As to adhere USB, thanks to dl324 now I know it will be just be a resettable fuse. A more expensive design maybe could differentiate different power source with different current capability...but the fuse will set off before telling if the current can be more..haha
 

Papabravo

Joined Feb 24, 2006
21,159
USB power supply support was one requirement. It must be a mobility issue and powerbanks was very synonymous. As for technical development with eyes on the user consumer I asked what if the device was plugged into the USB port of a desktop computer or laptop. As to adhere USB, thanks to dl324 now I know it will be just be a resettable fuse. A more expensive design maybe could differentiate different power source with different current capability...but the fuse will set off before telling if the current can be more..haha
I believe you are embarked on a path with unreasonable requirements and expectations. Ignore my advice if you wish, but don't be surprised if you eventually come to the same conclusion.
 

dl324

Joined Mar 30, 2015
16,845
now I know it will be just be a resettable fuse.
It's more than that. Unless your device negotiates with the host for more than 1 USB load, you're not allowed to use more. If your device needs more and can't negotiate, you shouldn't use the USB connector.

The automatic reset from overload is subject to failure. I would never plug a non-compliant USB device into a computer USB port.

A non-compliant USB host could be damaged by a non-compliant USB device.
 

ian field

Joined Oct 27, 2012
6,536
What dl324 is trying to say is that you are running way to close to the edge. USB cables are not meant to deliver power to motors and running right up to the limit has serious implications for degrading the user experience. Rethink your strategy -- it will result in superior performance and customer satisfaction.
My USB external hard drive has a special lead with 2 plugs to pick up the current it needs.

Might be an option if the TS can find a source of aftermarket split USB leads.
 

Thread Starter

muhzd

Joined May 25, 2009
34
I'm understanding now.
Actually the motored gadget implements a rechargeable battery. I'll limit the use of USB power for charging purposes only and not for power. As you expected, it's a loss of feature.

but then, you need to negotiate with the host PC for more than 1 USB device current(100mA?)..you mean those 5V 1.2W portable flexible USB LED light @Amazon are making it?
This application note uses USB power for battery charging purposes but never mentioned talking to the USB host for more current.
 

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Thread Starter

muhzd

Joined May 25, 2009
34
@ian field
With, USB Y-Cable I can connect those to the collector and base of a transistor and the emitter would be indicative. Then a powerbank with only one output port wouldn't be useful. or when a USB hub is used.
 
Last edited:

Papabravo

Joined Feb 24, 2006
21,159
I'm understanding now.
Actually the motored gadget implements a rechargeable battery. I'll limit the use of USB power for charging purposes only and not for power. As you expected, it's a loss of feature.

but then, you need to negotiate with the host PC for more than 1 USB device current(100mA?)..you mean those 5V 1.2W portable flexible USB LED light @Amazon are making it?
This application note uses USB power for battery charging purposes but never mentioned talking to the USB host for more current.
I can't say for sure what an individual product may or may not do. It may work as intended with a non-compliant host. What we do know is the procedure defined in the USB 2.0 specification which is publicly available. If you comply with the specification then your device will maximize the probability of working with any host. If you don't your customer is on their own when the plug your device in. In my experience customers are not fond of devices they've purchased working sometimes and not others.
 

Thread Starter

muhzd

Joined May 25, 2009
34
It sure is a dilemma to provide a perfect product.
Now I know why my when my smartphone is connected to the PC's USB, multiple connection indication is resulted. I thought the physical contact was poor. USB is very generalized nowadays.
The best fool proof method I can think of right now is to put an indication like a sticker for USB current requirements and maybe with an extra emphasis on USB ports of the desktop PC or laptop. Plus now I can capitalize on higher current capabilities of powerbanks for instance for higher charge rate for the battery.

well, next is posting the circuit i came up with. i myself think something is funny. it'll be in the The Projects Forum section.
 

dl324

Joined Mar 30, 2015
16,845
you need to negotiate with the host PC for more than 1 USB device current(100mA?)..you mean those 5V 1.2W portable flexible USB LED light @Amazon are making it?
Individuals who are ignorant of the USB spec, or don't care about compliance, can make devices that don't play well with USB hosts. Any device that can't negotiate with a USB host for current above one load is called a USB Decoration and isn't allowed to draw more than one load.

Do people violate the USB Spec? Of course. If you're ignorant of the spec, you don't know any better. If you know about the spec and violate it, you're worse than ignorant.
This application note uses USB power for battery charging purposes but never mentioned talking to the USB host for more current.
That application doesn't say anything about connecting to a USB host; only using the standard charger connector:
upload_2017-7-5_9-20-19.png
 

Thread Starter

muhzd

Joined May 25, 2009
34
Thanks dl324. I really appreciate it. I need all the help I can get. I'm quite new in the business and hope can maintain it. I'm finishing my master studies and will attend convocation this november. Meanwhile I got this order and labelled myself freelance. Most of the stuff are new and isn't a straight forward build. I really can't tell if anything was upside down without a straight face truthfully asking others to confirm.

Btw, the mosfet symbol before the inductor in the previous application note figure 3 should be the other way around right?
 
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