8.2.2.10 Compensation Component Selection

There are several industry techniques used to compensate DC/DC regulators. The method presented here is easy to calculate and yields high phase margins. For most conditions, the regulator has a phase margin between 60° and 90°. The method presented here ignores the effects of the slope compensation that is internal to the TPS54320. Since the slope compensation is ignored, the actual crossover frequency is usually lower than the crossover frequency used in the calculations. Use SwitcherPro software for a more accurate design.

Type III compensation is used to achieve a high-bandwidth, high-phase margin design. This design targets a crossover frequency (bandwidth) of 48 kHz (1/10 of the switching frequency). Using Equation 32 and Equation 33, the power stage pole and zero are calculated at 6.46 and 1778 kHz, respectively. For the output capacitance, C_O, use a derated value of 22.4 µF.

Equation 32.

Equation 33.

Now the compensation components can be calculated. First, calculate the value for R4 which sets the gain of the compensated network at the crossover frequency. Use Equation 34 to determine the value of R4.

Equation 34.

Next calculate the value of C4. Together with R4, C4 places a compensation zero at the modulator pole frequency. Use Equation 35 to determine the value of C4.

Equation 35.

Using Equation 34 and Equation 35, the standard values for R4 and C4 are 1.78 kΩ and 0.015 µF. The next higher standard value for C4 is selected to give a compensation zero that is slightly lower in frequency than the power stage pole.

To provide a zero around the crossover frequency to boost the phase at crossover, a capacitor (C11) is added parallel to R8. The value of this capacitor is given by Equation 36. The nearest standard value for C11 is 100 pF.

Equation 36.

Use of the feed-forward capacitor, C11, creates a low-AC impedance path from the output voltage to the VSENSE input of the IC that can couple noise at the switching frequency into the control loop. TI does not recommend use of a feed-forward capacitor for high-output voltage ripple designs (greater than 15-mV peak to peak at the VSENSE input) operating at duty cycles of less than 30%. When using the feed-forward capacitor, C11, always limit the closed loop bandwidth to no more than 1/10 of the switching frequency, ƒsw.

An additional high-frequency pole can be used if necessary by adding a capacitor in parallel with the series combination of R4 and C4. Equation 37 gives the pole frequency. This pole is set at roughly half of the switching frequency (of 480 kHz) by use of a 330-pF capacitor for C6. This helps attenuate any high-frequency signals that might couple into the control loop.

Equation 37.