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

MP2355 Rev. 1.5

www.MonolithicPower.com

9

5/1/2006

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© 2006 MPS. All Rights Reserved.

TM

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

L1

C2

R3

C3

C6

2.5V

10µH

min.

22µF

Ceramic

3.9kΩ 5.6nF

None

3.3V

15µH

min.

22µF

Ceramic

4.7kΩ 4.7nF

None

5V

15µH

min.

22µF

Ceramic

7.5kΩ 2.7nF

None

12V

22µH

min.

22µF

Ceramic

15kΩ

1.5nF

None

2.5V

10µH

min.

560µF Al.

30mΩ ESR

100kΩ

1nF

150pF

3.3V

15µH

min.

560µF Al

30mΩ ESR

120kΩ

1nF

120pF

5V

15µH

min.

470µF Al.

30mΩ ESR

150kΩ

1nF

82pF

12V

22µH

min.

220µF Al.

30mΩ ESR

169kΩ

1nF

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:

FB

OUT

CS

EA

C

V

V

G

G

f

2

C

2

3

R

×

×

×

×

π

=

2) Choose the compensation capacitor (C3) to

achieve

the

desired

phase

margin.

For

applications with typical inductor values, setting

of the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C

f

3

R

2

4

3

C

×

×

π

>

Where R3 is the compensation resistor value and

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:

2

f

R

2

C

2

1

S

ESR

<

×

×

π

If this is the case, then add the second

the location of the ESR zero. Determine the C6

value by the equation:

3

R

R

2

C

6

C

ESR

×

=

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.

MP2355

SW

BS

10nF

5V

MP2355_F03

Figure 3—External Bootstrap Diode

This diode is also recommended for high duty

cycle operation (when

IN

OUT

V

V

>65%) and high