AP6502A

Document number: DS35812 Rev. 4 - 2

9 of 14

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August 2015

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AP6502A

amplifier voltage gain.

The control loop transfer function incorporates two poles, one is due to the compensation capacitor (C3) and the output resistor of error amplifier,

and the other is due to the output capacitor and the load resistor. These poles are located at:

VEA

EA

P1

A

3

C

2

G

f









LOAD

P2

R

2

C

2

1

f









One zero is present due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at:

3

R

3

C

2

1









The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where

the feedback loop has the unity gain is crucial.

A rule of thumb is to set the crossover frequency to below one-tenth of the switching frequency. Use the following procedure to optimize the

compensation components:

1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation:

FB

OUT

CS

G

EA

FB

OUT

CS

EA

V

V

G

fs

1

.

0

2

C

2

V

V

G

G

fc

2

C

2

3

R



























crossover frequency to provide sufficient phase margin. Determine the C3 value by the following equation:

fc

3

R

2

3

C









Where R3 is the compensation resistor value.

(V)

(µF)

(µF)

(kΩ)

(nF)

L1

(µH)

1.2

22

47

3.24

6.8

3.3

1.8

22

47

6.8

6.8

3.3

2.5

22

47

6.8

6.8

10

3.3

22

47

6.8

6.8

10

5

22

47

6.8

6.8

10

12

22

47

6.8

6.8

15

Table 2. Recommended Component Selection