Electronic – Likely consequences of slightly exceeding 500mA USB current

current-limitingusbusb device

As we know, a USB 2.0 device, once it has properly negotiated with the host, is allowed to draw 500mA.

Suppose that you are designing such a device, and your input current limiter chip is tuned by a resistor. Let's say that the resistors available to you are ±1% tolerance.

Should you size your resistor to draw 500mA in the nominal case, knowing that in the maximum case you will draw 505mA? Or should you size your resistor to draw 495mA, so that you are guaranteed to respect the exact limit, but less power is available to your device?

Best Answer

The current limit detection by a host mode device that emits current to a USB device may or may not be accurate. Here are the types of host devices that you are likely to find:

  1. Some host devices such as a PC may have no limiting on the port itself. In such case a big overload of several Amps on the port may instead cause the internal 5V supply to shut down.
  2. Other devices may use a conventional type of fuse that is selected to blow out at some load current over 500mA. That fuse may be rated at say 0.75 or 1.0 ampere and blow at a level above that depending upon the fuse characteristics.
  3. It is common to find usage of poly-fuse type devices to limit the current. Similar to conventional fuses on the ratings side they will trip at a level quite a bit higher than the 500mA level. The advantage of this type is that they will reset when the overload is removed and the poly-fuse cools down. A downside of a poly-fuse design is that the trip current level generally increases when the device has tripped one or more times.
  4. Finally there are host mode USB ports that are equipped with 5V USB switch parts. These are designed to support actually turning the port power on and off. Such devices contain actual load current sensors that can be more accurate than the fuse types described above. But to keep part cost low the sensors are not more accurate than say a 20% to 40% overload.

No USB host mode device designer wants to make a device that nuisance trips and causes grief for their users. For this reason it will be rare to find a port that supports USB power source cut off at 501mA. Instead the cutoff, if any, will be much higher from say 600mA and possibly up to the current limit of the internal 5V supply.

An interesting point is to open up the typical powered USB four port hub. It is not uncommon to find the internal circuit board designed to use the 5V port power switch parts. However likely as not the parts may not even be installed and instead be replaced by a direct connection to the 5V supply. The manufacturer may then use the same circuit board in several models of their USB hubs that are offered at two different price points. As you can imagine the USB hub market place is an extremely competitive one and every extra part in the unit lowers the profit made on each sale.

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