7.4.3 Small Signal Model for Frequency Compensation

The TPS54678 uses a transconductance amplifier for the error amplifier and readily supports two of the commonly used frequency compensation circuits. The compensation circuits are shown in Figure 29. The Type 2 circuits are normally implemented in high-bandwidth power supply designs using low ESR output capacitors. In Type 2A, one additional high-frequency pole is added to attenuate high-frequency noise.

Figure 29. Types of Frequency Compensation

The design guidelines for TPS54678 loop compensation are as follows:

1. Set up crossover frequency fc.
2. R3 can be determined by Equation 10:
Equation 10.
where
• gm_ea is the GM amplifier gain,
• gm_PS is the power stage gain (20 A/V).
3. Place a compensation zero at the dominant pole C1 can be determined by Equation 11:
Equation 11.
4. C2 is optional. It can be used to cancel the zero from ESR of the Co in Equation 12:
Equation 12.