I'm a SW head trying to design a circuit that can switch 5V to a USB Port for embedded Host Operation and switch it off for USB Device operation. Microchip have a few schematics which show the use of their MCP1253 Charge pump for this.

Obviously they know what they're doing but looking at the specs for this device it can supply 150mA of Output current. That seems very light weight for USB Host? In addition it provides a regulated output power but since my input Voltage would be 5V power rail of the board I'm not sure I need a regulator. Obviously I'm missing something.

I'd be very grateful if somebody could explain why this is used to power USB Host at such low current.

  • 1
    \$\begingroup\$ You didn't reference the circuit, but they may be starting with 3.3 V or something else other than the 5 V required for USB. They may also be trying to limit the current. Charge pumps are usually forgiving of output shorts and then draw only a finite predictable current from the source. 150 mA is plenty for something like a flash drive. If your USB device needs more power, then you need a different solution. Why not a PFET and polyfuse in series with your 5 V supply? \$\endgroup\$ – Olin Lathrop Jan 22 '14 at 19:32

A USB 2.0 host is only guaranteed to be able to supply 100mA (one unit load) without negotiation, so 150mA is just fine to allow any device to enumerate (in theory).

Of course there are all kinds of things that people could plug into a USB port that don't honor the specification and draw hundreds of mA, so some customers might be disappointed that their coffee cup warmer and 4,000 RPM USB LED fan doesn't work.

Some compliant devices ("low power") will operate from a single 100mA maximum unit load. Some devices "high power) require more than 100mA to operate, and such devices (if compliant) would enumerate but probably would not be able to operate properly with the 150mA limit. I believe external powered hubs are always less than one unit load.

Anyway, obviously you can't design a circuit until you determine the requirements such as the maximum number of unit loads you are going to allow for, how compliant to the standard you're going to be etc.

As far as compliance goes, battery powered Root Port hubs are allowed to limit their supply current to one unit load (100mA) and still be compliant. That's called a "low power" port. Otherwise, you have to be prepared to pony up to five unit loads (0.5A) upon polite request.

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