MP2403 – 3A, 32V, 250kHz INTEGRATED SYNCHRONOUS STEP-DOWN CONVERTER

MP2403 Rev.1.0

www.MonolithicPower.com

10

4/20/2010

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© 2010 MPS. All Rights Reserved.

In this case, a third pole set by the

compensation

capacitor

(C6)

and

the

compensation

resistor

(R3)

is

used

to

compensate the effect of the ESR zero on the

loop gain. This pole is located at:

3

R

6

C

2

1

P

×

×

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

important.

Lower crossover frequencies result in slower

line and load transient responses, while higher

crossover frequencies could cause system

unstable. A good rule of thumb is to set the

crossover frequency to approximately one-tenth

of the switching frequency. Switching frequency

for the MP2403 is 250KHz, so the desired

crossover frequency is 25KHz.

Table 3 lists the typical values of compensation

components for some standard output voltages

with various output capacitors and inductors. The

values of the compensation components have

been optimized for fast transient responses and

good stability at given conditions.

To optimize the compensation components for

conditions not listed in Table 2, the following

procedure can be used.

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine the

R3 value by the following equation:

FB

OUT

CS

EA

C

V

V

G

G

f

2

C

2

3

R

×

×

×

×

π

=

25KHz.

2. Choose the compensation capacitor (C3) to

achieve

the

desired

phase

margin.

For

applications with typical inductor values, setting

the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C

f

3

R

2

4

3

C

×

×

π

>

Table 3—Compensation Values for Typical

Output Voltage/Capacitor Combinations

L

C2

R3

C3

C6

1.8V

4.7μH

100μF

Ceramic

5.6kΩ

3.3nF

None

2.5V

4.7-

6.8μH

47μF

Ceramic

4.7kΩ

4.7nF

None

3.3V

6.8-

10μH

22μFx2

Ceramic

5.6kΩ

3.3nF

None

5V

10-

15μH

22μFx2

Ceramic

7.5kΩ

3.3nF

None

12V

15-

22μH

22μFx2

Ceramic

10kΩ

1.2nF

None

1.8

4.7μH

100μF

SP-CAP

10kΩ

2.2nF 100pF

2.5V

4.7-

6.8μH

47μF

SP-CAP

5.6kΩ

3.3nF

None

3.3V

6.8-

10μH

47μF

SP-CAP

6.8kΩ

2.2nF

None

5V

10-

15μH

47μF

SP CAP

10kΩ

2.2nF

None

2.5V

4.7-

6.8μH

560μF Al.

30mΩ ESR

10kΩ

7.5nF

1.5nF

3.3V

6.8-

10μH

560μF Al

30mΩ ESR

10kΩ

10nF

1.5nF

5V

10-

15μH

470μF Al.

30mΩ ESR

15kΩ

7.5nF

1nF

12V

15-

22μH

220μF Al.

30mΩ ESR

15kΩ

10nF

390pF

3. Determine if the second compensation

capacitor (C6) is required. It is required if the

ESR zero of the output capacitor is located at

less than half of the 250KHz switching

frequency, or the following relationship is valid:

2

f

R

2

C

2

1

S

ESR

<

×

×

π

If this is the case, then add the second

at the location of the ESR zero. Determine the

C6 value by the equation:

3

R

R

2

C

6

C

ESR

×

=