MP29296 – 2A, 23V SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER

MP29296 Rev. 1.7

www.MonolithicPower.com

8

2/5/2010

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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 instability. A good rule of thumb is to set

the crossover frequency below one-tenth of the

switching frequency.

To optimize the compensation components, 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

S

FB

OUT

CS

EA

C

V

V

G

G

f

1

.

0

2

C

2

V

V

G

G

f

2

C

2

3

R

×

×

×

×

×

π

<

×

×

×

×

π

=

which is typically below one tenth of the

switching frequency.

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 the C3 value by the following equation:

C

f

3

R

2

4

3

C

×

×

π

>

Where R3 is the compensation resistor.

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 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

×

=

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.

MP29296

SW

BS

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

output

voltage

applications