implement pwm current limit

[asmboy]

the circuit you show ought to work and is what i intended.
i'd use a .1uf cap and go up if it s too touchy under drive .
(monitor the collector waveform to see )

The 220 ohm resistor is appx 10x the sampling resistor.
It protects against wildly excess base current under worst case conditions,
without messing up the effective beta of the circuit.
Larger values will begin to degrade, or "degenerate" the beta of the circuit

Vsat for the transistor is not relevant to the task, as the intended
design avoids saturation as an operating mode.
This is because the current will be shut off on any PWM cycle that has enough peak current to turn on the transistor.

The capacitor is meant to add a slow saw tooth "hiccup" kind of protection
that keeps a total current in a "safe area" that won't harm either the bridge or motor during a dead-locked stall.

For this reason it is in your best interests to ramp the PWM rate up when starting up from a fixed position.
You can always safely go from some level of PWM down to zero w/o a ramp however.

Anyway......
Enough collector current will flow when Isense =2amps
to pull down the collector to less than .6v which will disable the arms
of the bridge, and based on th capacitor value- set the sawtooth frequency.

How do i know it ought to work?
Because i have been designing circuits for a living
since about 1970 or so.
PICs are much newer to me than solder as a programming method .