AP5101

1.5A Step-Down Converter with 1.4MHz Switching

Frequency

AP5101

Document number: DS32258 Rev. 1 - 2

10 of 15

www.diodes.com

July 2010

© Diodes Incorporated

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

CS

G

EA

G

fs

1

.

0

2

C

2

FB

V

OUT

V

CS

G

EA

G

fc

2

C

2

3

R

×

×

×

×

×

π

<

×

×

×

×

π

=

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.

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

And

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

+

=

Peak current determines the required saturation current rating, which influences the size of the inductor. Saturating the

inductor decreases the converter efficiency while increasing the temperatures of the inductor, the MOSFET and the diode.

Hence choosing an inductor with appropriate saturation current rating is important.

A 1µH to 10µH inductor with a DC current rating of at least 25% percent higher than the maximum load current is

recommended for most applications.

For highest efficiency, the inductor’s DC resistance should be less than 200m

Ω. Use a larger inductance for improved

efficiency under light load conditions.