MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP1570 Rev. 1.5

www.MonolithicPower.com

8

1/31/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

TM

The system may have another zero of

importance, if the output capacitor has a large

capacitance and/or a high ESR value. The zero,

due to the ESR and capacitance of the output

capacitor, is located at:

ESR

ESR

R

2

C

2

1

f

×

×

π

=

In this case (as shown in Figure 2), 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 MP1570 is 340KHz, so the desired

crossover frequency is 34KHz.

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.

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

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

=

34KHz.