USB 2.0 Compliance
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| Posted on April 21, 2015 at 20:28:11 | ||
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Posts: 1101
Joined: December 25, 2014 |
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. "Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
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| RE: USB 2.0 Compliance, posted on April 22, 2015 at 00:15:21 | |
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Posts: 4209
Location: Somewhere nice on planet dirt Joined: September 25, 1999 |
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""
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. Thor At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?
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