Buck - Closed Loop (step down)

Hi.

I saw a circuit - Buck regulator - Closed Loop, which uses PWM to produce regulated voltage. Image is attached.

I saw it also in some books.

The second op-amp is feeded (in one of it's entries) with sawtooth signal, but I couldn't understand its purpose. What is it's (the sawtooth) purpose? I can't see.

Thank you.

The second opamp should be thought of as a comparator. The error signal from the feedback amplifier is compared to the sawtooth to form a pulse width modulated pwm signal.

I understand that.
I'll be more specific:

When the sawtooth is lower than the (+Vcc) of the first op-amp, we get that the sawtooth has no influence.

When the sawtooth is higher than the (+Vcc) of the first op-amp, we get that the second op-amp's output is it's (+Vcc).

Well, that's clear. But what does it help?
I'll rephrase: In a good design, what will be the +Vcc of the second op-amp, and what will be the sawtooth's frequency?

Your first opamp should have some frequency compensation components, Resistors and Capacitors to stabilize the loop. for example, a resistor from
the output of buck regulator to the inverting node of the opamp and a capacitor from inverting node to the opamp output. This network forms a pole in the frequency domain.

Your TP9 voltage should be the mosfet gate drive voltage threshold plus the TP5 voltage. e.g. if your supply voltage (tp5) is 5V and the mosfet voltage is 5V, then the TP9 voltage should be greater than 10V.

In a practical ic design, the voltage supply to the opamp and the comparator would be the same voltage.
The pwm ramp valley would be greater than zero volts for noise immunity and so that the converter could have a zero duty cycle and skip pulses. The valley and the peak of the ramp would be within the common mode range for the amplifier and comparator.

Also, the oscillator would initiate the turn on of the mosfet through a SR latch and initiate the ramp at the valley voltage. The intersection of the error voltage and the ramp would turn off of the mosfet. The output of the comparator is the pwm signal. The pwm signal is feed to the SR latch.
Frequency of the ramp is the switching freqeuency of the power supply.
The frequency is chosen based on the specfications such as output voltage ripple, inductor ripple current.

have u run the circuit? coz i really need help on how to run the circuit and see the output...