Push Pull Switch Mode Power Supplies
The working of the circuit is described here in this document which describes the functionality of PWM modulators.

RT and Ct are part of an oscillator circuit where


  \$ f = \frac{1.18}{R_T C_T}\$
  
  (Rt is in kΩ, Ct is in uF, f is in kHz)
  
  Practical values of CT fall between 1 nF and 100 nF. Practical values
  of RT fall between 1.8 kW and 100 kW. This results in a frequency
  range typically from 120 Hz to 500 kHz.


Source: datasheet listed above