AP65403

Document number: DS37126 Rev. 1 - 2

10 of 15

www.diodes.com

March 2014

© Diodes Incorporated

AP65403

Compensation Components

The AP65403 has an external COMP pin through which system stability and transient response can be controlled. COMP pin is the output of the

internal trans-conductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of

the control system. The DC gain of the voltage feedback loop is given by:

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

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

×

×

π

=

LOAD

P2

R

2

C

2

1

f

×

×

π

=

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

×

×

π

=

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

×

×

×

×

π

<

×

×

×

×

π

=

×

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

fc

3

R

2

3

C

×

×

π

>

Where R3 is the compensation resistor value.

VOUT

(V)

Cin/C1

(µF)

Cout/C2

(µF)

Rc/R3

(k

Ω)

Cc/C3

(nF)

L1

(µH)

2.5

44

72

10.5

6.8

4.7

3.3

44

72

10.5

6.8

4.7

5

44

72

10.5

6.8

4.7

12

44

72

10.5

6.8

6.5 - 10

Table 2. Recommended Component Selection

Inductor

Calculating the inductor value is a critical factor in designing a buck converter. For most designs, the following equation can be used to calculate

the inductor value;

SW

L

IN

OUT

IN

OUT

f

∆I

V

)

V

(V

V

L

⋅

⋅

−

⋅

=

Where

L

f

is the buck converter switching frequency.