PWM inside linear feedback loop

Hi
I'm attempting to control a brushed DC motor in troque control with an analogue pwm chip. I have the chip inside a feedback loop with a simple
proportional controller and a current sensor providing feedback. My demand is therefore current.
I've been simulating the loop using simulink. I have a problem in that increasing the proportional gain such that I get a decent step response causes the input to the pwm IC to follow the saw-tooth reference signal which in turn causes strange things to happen to the pwm output signal.
Is this just the limiting factor on the loop gain of a pwm system...or am i doing something stupid?
Cheers for any advice in advance
Steve Minshull
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