The loop bandwidth and phase margin determines the response to load transients, while ensuring that the output noise level meets the requirements. A common choice of loop unity gain frequency is 5% of the switching frequency. This is simple to compensate, low noise and provides sufficient transient response for most applications. The plant bode plot is examined for gain and phase at the desired loop unity-gain frequency and the compensator is designed to adjust the loop gain and phase to meet the intended loop unity gain frequency and phase margin (typically about 55°). When gain is required, the ratio of R_COMP and R_{FB_TOP} sets the error amplifier to provide the correct amount.

Equation 23.

where (in reference to Figure 8-5):

• R_COMP is R4
• R_{FB_TOP} is R5

The phase margin is boosted by a transfer function zero at frequency given by Equation 23 and a pole at frequency given by Equation 24.

Equation 24.

where (in reference to ):

• C_COMP is C6

Equation 25.

where (in reference to Figure 8-5):

• C_HF is C5

The low frequency pole is determined with Equation 26.

Equation 26.

where (in reference to Figure 8-5):

• A_VOL is the open loop gain of the error amplifier

Optimal regulation is achieved by setting F_POLE(LOW) as high as possible, but still permitting F_ZERO to insure the desired phase margin. The feedback resistors (R_{FB_TOP} and R_{FB_BOTTOM}) are chosen to be 10.2 kΩ and 3.4 kΩ respectively. These values produce a feedback signal that has a desirable signal to noise ratio. F_ZERO and F_POLE(HIGH) are set to be 450 Hz and 25.5 kHz respectively. Based on these values, R_COMP, C_COMP, and C_HF are chosen to be 7.5 kΩ, 0.47 µF, and 820 pF respectively. These values produce a crossover frequency of approximately 3 kHz with a phase margin of 60°.