AP6501

Document number: DS36059 Rev. 1 - 2

10 of 15

www.diodes.com

September 2012

© Diodes Incorporated

AP6501

Applications Information (cont.)

Compensation Components (cont.)

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

A

3

C

2

EA

G

P1

f

×

×

π

=

LOAD

R

2

C

2

1

P2

f

×

×

π

=

Where GEA is the error amplifier trans-conductance.

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

fZ1

×

×

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

V

OUT

V

EA

G

fs

1

.

0

2

C

2

FB

V

OUT

V

CS

G

EA

G

fc

2

C

2

3

R

CS

G

×

×

×

×

π

<

×

×

×

×

π

=

×

Where fC is the crossover frequency, which is typically less than one tenth of the switching frequency.

2. Choose the compensation capacitor (C3) to achieve the desired phase margin set the compensation zero, fZ1, to below one fourth of the

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)

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

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

f

L

ΔI

IN

V

)

OUT

V

IN

(V

OUT

V

L

⋅

⋅

−

⋅

=

Where

L

ΔI is the inductor ripple current.

And SW

f

is the buck converter switching frequency.

Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor peak current is calculated from:

2

L

ΔI

LOAD

I

L(MAX)

I

+

=