I believe, in many cases, the "hysteretic regulator" generates the PWM directly, at least for a buck type regulator. That is, when the voltage drops below the lower voltage threshold, it turns the switch on, and when the voltage rises above the upper voltage threshold, it turns the switch back off. The frequency of this PWM depends upon the load, the voltage delta, and the inductance and capacitance of the output filter.In most cases, a "hysteretic regulator" is a switcher that operates in burst mode, which is to say turns the PWM driver on when the output voltage decays to a lower threshold level and the switcher turns off when it reaches the upper voltage threshold.
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by Aaron Carman