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.
Dedicated chip advantages
- Does its job without external intervention.
- Doesn't need a regulated supply already to run. Runs directly from the raw input supply.
- High speed, since everything is in dedicated logic.
- 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.
- Very little additional circuitry may be required. For the most integrated chips, all you need to add is the inductor and output capacitor.
- Cheaper than a microcontroller.
Microcontroller running switching power supply advantages
- Cheaper and lower footprint if a microcontroller with a spare PWM module is already there anyway.
- 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.
- Easy to implement high level management, telemetry, LIN, IIC, drive a display, etc.
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.