Electronic – Bootstrapped charge pump design for phase frequency detector

If you look carefully, you will see that C1 doesn't get discharged in the second circuit. After the first positive pulse, it gets charged up, then mostly stays that way. Since the voltage on C1 is then roughly constant, you see the same square wave at Out as you do on the emitters, except that it is offset by the voltage on C1. Note that Out varies from about 0 to negative. This is the same signal you would see on the right side of C1 in the first circuit. The addition of D2 and C2 take the 0 to negative pulses and make a flat negative. You can think of D2 and C2 as being a negative peak detector.

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.