The transfer function from the control to the output of the step-down converter, where VRAMP represents the peak to peak value of the sawtooth wave at the in-phase input end of PWM generator U1, and R represents the output resistance, which is usually equivalent to the load resistance. C is the output capacitance.

    The transfer function of the feedback part of the buck converter can be divided into two types: traditional error amplifier and transconductance amplifier. The feedback resistance ratio of transconductance amplifier determines the gain and phase, and the same resistance ratio has the same gain and phase. In traditional operational amplifiers, the upper resistance affects the amplitude phase curve. To change the voltage, it is better to keep the upper resistance unchanged and change the lower resistance.

    The closed-loop transfer function of the step-down converter is: when the open-loop transfer function G (s) H (s) is - 1, the closed-loop transfer function will be infinite, that is, the phase of 0db in the Bode diagram of the amplitude of the open-loop threading function cannot be 180 °, otherwise the system will be unstable and cause oscillation. To ensure that the system will not oscillate when disturbed, certain phase margin and amplitude margin shall be reserved. Generally, the phase is set to 45 °, because under the 45 ° phase margin, the typical step response has only two rings, which can give consideration to both stability and response speed. Most of the current buck ICs do not reserve compensation interfaces externally, and the error amplifier will perform internal compensation. Since the compensation parameters are unknown, the feedback resistance should be selected according to the recommended parameters as far as possible. After the power supply is designed, the response of the power supply to the output dynamic load must be tested to observe the response speed and whether it is generated by ringing.