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I'll assume by "PMIC" you mean a dedicated switching power supply chip.  Such a chip contains the PWM generator and receives output voltage feedback at a minimum.  They may also include a driver for an external FET as the switching element, include the switching element directly, include the diode with synchronous rectification circuitry around it, maybe a shutdown input, a power-good output, and other additional features.

<h3>Dedicated chip advantages</h3><ol>

<li>Does its job without external intervention.

<li>Doesn't need a regulated supply already to run.  Runs directly from the raw input supply.

<li>High speed, since everything is in dedicated logic.

<li>Built-in compensation.  As long as you use the right parts according to the datasheet, stability, transient response, regulation performance, and other parameters have already been tweaked for you.

<li>Very little additional circuitry may be required.  For the most integrated chips, all you need to add is the inductor and output capacitor.

<li>Cheaper than a microcontroller.

</ol>

<h3>Microcontroller running switching power supply advantages</h3><ol>

<li>Cheaper and lower footprint if a microcontroller with a spare PWM module is already there anyway.

<li>Much more flexible.  Since the controller is in firmware, it can do more than is reasonable in dedicated silicon.  Examples include feed-forward of the input voltage, non-linear control over parts of the range, run-time tweaking of operational parameters based on measurements, etc.

<li>Easy to implement high level management, telemetry, LIN, IIC, drive a display, etc.

</ol>

<h3>General tradeoffs</h3>

If you just want a 5.0 V or 3.3 V power rail to run stuff on your board from maybe a 9 to 24 V input, then a dedicated switcher chip is usually the best choice.  They are small, cheap, and just work.

If you are implementing a power supply as an end onto itself, then something more sophisticated than a dedicated chip is often useful.  High end power supplies with 10s of Watts or more output require enough other parts that the difference in cost from a switcher chip to a micro is minimal in the overall scheme.  Even if you just implement a controller with standard compensator, you may want telemetry, external digital control, driving of status indicators, electronic fuse, etc.

I have had designs where both dedicated chips and a microcontroller were used to implement different supplies.  In several recent designs, I've used dedicated chips to make the 3.3 V and 5.0 V supplies from the industrial 24 V power bus, and a spare PWM generator in the micro to implement an isolated supply.

The PWM output controlled the switch driving the input of a small transformer.  The resulting voltage on the isolated side was compared to a threshold, and the result sent back via opto-isolator.  That drove the shutdown input to the PWM.  Once set up in firmware, it runs completely in hardware.

This was basically a pulse on demand scheme.  Those result in more ripple than more sophisticated control schemes, but still good enough for many purposes.  They are also inherently stable with good transient response.