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(

)

C11

R8 fc

× ×

ESR

C6 =

C4 =

p =

VOUT

R4 =

Vref

´ ¦

Vref

VOUT

Coea

Roea

gmea

COMP

VSENSE

Type 2A

Type 2B

C11

Type 3

TPS54821

SLVSB14B – OCTOBER 2011 – REVISED FEBRUARY 2016

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Device Functional Modes (continued)

7.4.14 Small Signal Model for Frequency Compensation

The device uses a transconductance amplifier for the error amplifier and readily supports two of the commonly

used Type II compensation circuits and a Type III frequency compensation circuit, as shown in Figure 28. In

Type 2A, one additional high frequency pole, C6, is added to attenuate high frequency noise. In Type III, one

additional capacitor, C11, is added to provide a phase boost at the crossover frequency. See Designing Type III

Compensation for Current Mode Step-Down Converters (SLVA352) for a complete explanation of Type III

compensation.

The design guidelines below are provided for advanced users who prefer to compensate using the general

method. The below equations only apply to designs whose ESR zero is above the bandwidth of the control loop.

This is usually true with ceramic output capacitors. See the Application Information section for a step-by-step

design procedure using higher ESR output capacitors with lower ESR zero frequencies.

Figure 28. Types of Frequency Compensation

The general design guidelines for device loop compensation are as follows:

1. Determine the crossover frequency, fc. A good starting point is 1/10th of the switching frequency, fsw.

2. R4 can be determined by:

(14)

Where:

gmea is the GM amplifier gain (1300μA/V)

gmps is the power stage gain (21A/V)

Vref is the reference voltage (0.6V)

3. Place a compensation zero at the dominant pole:

C4 can be determined by:

(15)

4. C6 is optional. It can be used to cancel the zero from the ESR (Equivalent Series Resistance) of the output

capacitor Co.

(16)

5. Type III compensation can be implemented with the addition of one capacitor, C11. This allows for slightly

higher loop bandwidths and higher phase margins. If used, C11 is calculated from Equation 17.

(17)

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