Important notes on current consumption in USB2.0 interface

 

In embedded systems products, power consumption is a critical criteria. While power consumption is one part, lowest possible power and power in various states is a requirement which helps any interface to keep it's power consumption in check. Let us look at various power scenarios of USB2.0 interface.

 - When a device connected to host is in suspend mode, the specification limits the current draw to <= 2.5mA. In suspend state, host is not actively communicating with the device. This is a USB power saving scheme and device suspend state is automatically triggered after 3 ms of no activity on the USB. the wake up time from this suspend mode must be <= 10 ms.

Imagine a USB mouse:

• When idle for several minutes, the host stops polling it.
• The mouse enters suspend mode, drawing only ~500 µA.
• When you move the mouse, it sends a remote wake up signal.
• Host resumes communication, and the mouse returns to active mode.

- When no device is connected to host, the current must be 0mA which is understood as this is open state

- Current consumption of the device can be 100mA when the device is in active state and not configured yet by host. Devices must default to ≤100 mA when first plugged in.

 - As per USB2.0 specification, the maximum current draw from host is 500mA. 

The above rules apply to a data port and these days fast charging is the norm. Users want the device to be charged at a super speed. So, when no data is involved and it is only charging, the maximum current can go up to 1.5A.

Overall, for a standard USB2.0, power output can be as high as 2.5watts (5V*500mA). For charging, power output can be as high as 7.5watts (5V*1.5A).