Two ICs are used for this converter.  A dual op Amp generates a 20KHz square wave signal.  A triangle shape waveform is produced as part of that generator. That signal is fed to the non-inverting side of a LM393 voltage comparator.  The 0v to 5v control input is level shifted and attenuated to match the triangle wave peak to peak voltage.  That DC voltage is fed to the inverting side of the LM393 comparator.  The comparator output is thus a TTL level signal, whose pulse width can be controlled by the input DC voltage. With a 0v input, the duty cycle of the PWM waveform is 0%.  With a voltage of +5v, the duty cycle shifts to 100%. With the component values selected, the PWM frequency is 20KHz but can easily be changed by changing the resistor value at the R4 position.