MP1593 – 3A, 28V, 385KHz STEP-DOWN CONVERTER

MP1593 Rev. 1.9

www.MonolithicPower.com

9

9/14/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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

3.9kΩ

5.6nF

None

3.3V

6.8-

10µH

22µFx2

Ceramic

5.6kΩ

8.2nF

None

5V

10-

15µH

22µFx2

Ceramic

7.5kΩ

10nF

None

12V

15-

22µH

22µFx2

Ceramic

10kΩ

3.3nF

None

1.8

4.7µH

100µF

SP-CAP

5.6kΩ

3.3nF 100pF

2.5V

4.7-

6.8µH

47µF

SP-CAP

4.7kΩ

5.6nF

None

3.3V

6.8-

10µH

47µF

SP-CAP

6.8kΩ

10nF

None

5V

10-

15µH

47µF

SP CAP

10kΩ

10nF

None

2.5V

4.7-

6.8µH

560µF Al.

30mΩ ESR

10kΩ

5.6nF

1.5nF

3.3V

6.8-

10µH

560µF Al

30mΩ ESR

10kΩ

8.2nF

1.5nF

5V

10-

15µH

470µF Al.

30mΩ ESR

15kΩ

5.6nF

1nF

12V

15-

22µH

220µF Al.

30mΩ ESR

15kΩ

4.7nF 390pF

To optimize the compensation components for

conditions not listed in Table 3, the following

procedure can be used.

1. Choose the compensation resistor (R3) to set

the desired crossover frequency. Determine R3

by the following equation:

FB

OUT

CS

EA

C

V

V

G

G

f

2

C

2

3

R

×

×

×

×

π

=

(which typically has a value no higher than

38KHz).

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 C3 by the following equation:

C

f

3

R

2

4

3

C

×

×

π

>

Where R3 is the compensation resistor value.

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 385KHz switching frequency, or the

following relationship is valid:

2

f

R

2

C

2

1

S

ESR

<

×

×

π

switching frequency. If this is the case, then add

the second compensation capacitor (C6) to set

Determine C6 by the equation:

3

R

R

2

C

6

C

ESR

×

=

the ESR value of the output capacitor and R3 is

the compensation resistor.

External Bootstrap Diode

It is recommended that an external bootstrap

diode be added when the system has a 5V fixed

input or the power supply generates a 5V output.

This helps improve the efficiency of the regulator.

The bootstrap diode can be a low cost one such

as IN4148 or BAT54.

MP1593

SW

BS

3

1

10nF

5V

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty

cycle operation (when

IN

OUT

V

V

>65%) and high