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Hoping someone help me with what a couple of questions about USB output current and compliance.
Am I correct in thinking that to be USB 2.0 compliant, a PC/ renderer should be able output 5V at 0.5A on its power line? If the PC/ renderer could manage only half that current, would it be 2.0 compliant? Finally, I have seen it stated that the ability to output a higher current lowers the output impedance - is this correct and does it follow that a reduced current would increase the output impedance?
Thanks in advance for any assistance.
Regards,
91.
Follow Ups:
Hi,
http://en.wikipedia.org/wiki/USB#Power
This summary is based on the USB Standard documents:
"The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V ± 5%) between the positive and negative bus power lines (VBUS voltage)."
"A unit load is defined as 100 mA in USB 1.x and 2.0, and 150 mA in USB 3.0. A device may draw a maximum of five unit loads (500 mA) from a port in USB 1.x and 2.0, or six unit loads (900 mA) in USB 3.0."
"There are two types of device: low-power and high-power. A low-power device (such as a USB HID) draws at most one-unit load, with minimum operating voltage of 4.4 V in USB 2.0. A high-power device draws, at most, the maximum number of unit loads the standard permits."
"Some devices, such as high-speed external disk drives, require more than 500 mA of current and therefore may have power issues if powered from just one USB 2.0 port: erratic function, failure to function, or overloading/damaging the port. Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power and data, the other for power only) to plug into a computer. With such a cable, a device can draw power from two USB ports simultaneously. However, USB compliance specification states that "use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral""
USB power standards
Specification: USB 1.x and 2.0
Current: 500 mA
Voltage: 5 V
Power: 2.5 W
Upshot, ANY Host USB Port must provide at least 500mA to be compliant with USB2.0 Spec.
That said, it seems many in audio view such standards as USB merely as polite advise, rather than binding.
I recently measured a USB DAC with a very high power rating headphone amplifier drawing 1.7A (yes, that is 3.4 times the absolute maximum allowed for USB2.0) during sine-wave testing at 0dBFS into a 16 Ohm headphone load.
I guess it would be fun to see how that DAC would be getting on with your source.
You can buy a quite cheapo USB Power tester that allows you to both test the USB Power source with some additional dummy loads and to test the current drawn by your DAC:
http://www.ebay.com/itm/OLED-USB-Charger-Capacity-power-Current-Voltage-Detector-Tester-Meter-with-Case-/121425835081
For dummy loads, cut up a cheap USB cable and solder in suitable load resistors (22Ohm/5W = 5V/230mA; 10Ohm/5W = 5V/500mA for USB 1/2 maximum load testing and 5.6Ohm/9Watt = 5V/900mA for USB 3).
> Finally, I have seen it stated that the ability to output a
> higher current lowers the output impedance - is this correct
> and does it follow that a reduced current would increase the
> output impedance?
Output impedance is generally a feature of the circuit design, regulator, USB power switch etc. and NOT directly related to how much current can be drawn.
Ciao T
At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to untolerable pain. Really, what more could we need?
Thanks very much for the informative reply Thorsten - much appreciated!
I did some more digging, including reviewing the the USB 2.0 compliance documents and found the current specification, which of course, is exactly has you have quoted.
I also suspected, but was not at all sure, of current having little to not effect on impedance - they are two different systems in the same connector.
There is one renderer/ music player manufacturer's web page that states they use a "dedicated USB Audio Class 2.0 output", which is marked as providing 5V at 0.2A... I am guessing their USB implementation does not comply with appropriate standards. Interesting.
Thanks again Thorsten.
Cheers,
91.
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