MP2354 – 2A, 23V, 380KHz STEP-DOWN CONVERTER

MP2354 Rev. 1.4

www.MonolithicPower.com

9

1/6/2006

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© 2006 MPS. All Rights Reserved.

TM

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 MP2354 is 380KHz, so the

desired crossover frequency is around 38KHz.

Table 4 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 4—Compensation Values for Typical

Output Voltage/Capacitor Combinations

L1

C2

R3

C3

C6

2.5V

10µH

min.

22µF

Ceramic

5.6kΩ 4.7nF

None

3.3V

15µH

min.

22µF

Ceramic

7.5kΩ 3.3nF

None

5V

15µH

min.

22µF

Ceramic

11kΩ

2.2nF

None

12V

22µH

min.

22µF

Ceramic

27kΩ

1nF

None

2.5V

10µH

min.

560µF Al.

30mΩ ESR

140kΩ

1nF

120pF

3.3V

15µH

min.

560µF Al

30mΩ ESR

187kΩ

1nF

82pF

5V

15µH

min.

470µF Al.

30mΩ ESR

237kΩ

1nF

56pF

12V

22µH

min.

220µF Al.

30mΩ ESR

267kΩ

1nF

22pF

To optimize the compensation components for

conditions not listed in Table 4, 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

forth 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.

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

at the location of the ESR zero.

Determine the C6 value by the equation:

3

R

R

2

C

6

C

ESR

×

=