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EV2355DN-00A

3A, 23V, 380KHz Step-Down Converter

Monolithic Power Systems 

TM

MP2355 – 3A, 23V, 380KHz STEP-DOWN CONVERTER

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 MP2355 is 380KHz, so the desired

crossover frequency is around 38KHz.

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

VOUT

2.5V

3.3V

5V

12V

2.5V

3.3V

5V

12V

L1

10µH

min.

15µH

min.

15µH

min.

22µH

min.

10µH

min.

15µH

min.

15µH

min.

22µH

min.

C2

22µF

Ceramic

22µF

Ceramic

22µF

Ceramic

22µF

Ceramic

560µF Al.

30mΩ ESR

560µF Al

30mΩ ESR

470µF Al.

30mΩ ESR

220µF Al.

30mΩ ESR

R3 C3

3.9kΩ 5.6nF

4.7kΩ 4.7nF

7.5kΩ 2.7nF

15kΩ 1.5nF

100kΩ 1nF

120kΩ 1nF

150kΩ 1nF

169kΩ 1nF

C6

None

None

None

None

150pF

120pF

82pF

39pF

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:

R3 = 2π × C2 × fC × VOUT

GEA × GCS VFB

2) Choose the compensation capacitor (C3) to

achieve the desired phase margin. For

applications with typical inductor values, setting

the compensation zero, fZ1, to less than one forth

of the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C3 >

4

2π × R3 × fC

Where R3 is the compensation resistor value and

fC is the desired crossover frequency, 38KHz.

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 380KHz switching frequency, or the

following relationship is valid:

1

< fS

2π × C2 × RESR 2

If this is the case, then add the second

compensation capacitor (C6) to set the pole fP3 at

the location of the ESR zero. Determine the C6

value by the equation:

C6 = C2 × RESR

R3

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.

5V

BS

MP2355

SW

10nF

MP2355_F03

Figure 3—External Bootstrap Diode

This diode is also recommended for high duty

cycle operation (when VOUT >65%) and high

VIN

output voltage (VOUT>12V) applications.

MP2355 Rev. 1.5

www.MonolithicPower.com

9

5/1/2006

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2006 MPS. All Rights Reserved.

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