Raspberry Pi 140 mA USB Power Limit

The proper operation of the USB port depends upon the current requirement of the devices attached to it. The earlier versions of Raspberry Pi have a limit of 140 mA per port by way of polyfuses. This is actually a very sound design idea, which works fine. If you need to use high-powered USB peripherals then the designers recommend an externally powered USB hub.

The purpose of the poly fuse protection is to stop a USB device from bringing down the whole power supply. In the case of a large PC or a mainframe, it is vital to include this in the design; after all, you would not want the mainframe in a bank to shut down just because someone plugged in a damaged keyboard. However, for a "big" child’s toy computer such as this, where without the keyboard and mouse it is useless, you could probably do away with the polyfuse, and just have one main fuse F3.

USB Circuit Diagram

This is a very basic standard design usually found on motherboards that use the LAN9512 USB controller chip. However, on computer motherboards, the USB voltage rail would have a dedicated voltage regulator, placed after the poly fuse, thus ensuring precisely +5 V to the USB hub at all times. Unfortunately, due to the cost parameters of the design, the 5 V rail connects directly to the main power rail through the polyfuse.

Polyfuses exhibit an operating resistance that can cause a voltage drop. Looking at the documentation of the miniSMDC014F, the component specification suggests that at 20 °C the fuse may have a resistance between 1.5 Ω to 6 Ω. For every 100 mA of current that is drawn, the relative voltage drop across the fuse is between 0.15 V to 0.6 V.

As you can see, a voltage drop of 0.6 V, in the worst-case scenario, could pose a problem for many USB devices, which would have to operate on 4.4 V. This is of course assuming that the input supply is at 5 V. However, usually there is already a 0.3 V drop due to a poorly performing adapter, thus the final voltage for the USB devices could be as low as 4.1 V, which is well below the USB specifications.

My Raspberry Pi board had 3.4 Ω fuses, and a quick look at replacement PTC polyfuses from different manufacturers revealed that I could purchase ultra-low resistance devices that would be ideal in this application. Obviously, these would be a better choice of components so I decided to order them and use those instead.

Removing the USB Current Limit

As an experienced designer, I know there are other ways to design this part of the circuit. It is possible to design the port control circuit in the manner shown above. The LAN9512 chip allows this configuration and works much better in this way. The advantage is that it saves a diode and a fuse. The circuit is more optimised and more flexible at the same time allowing a wider range of keyboards and mice to function.

A single 300 mA fuse is still well within the safety parameters, and provides a better load balance between the two ports. Mice tend to have a much lower power requirement than a keyboard, and the keyboard can use the surplus power, hence both peripherals can operate properly without tripping the fuse.

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