MP2358 – 2A, 23V, 370KHz STEP-DOWN CONVERTER

MP2358 Rev. 1.3

www.MonolithicPower.com

8

3/23/2006

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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 the system

to become unstable. A good rule of thumb is to

set the crossover frequency to 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

C

V

V

G

G

f

2

C

2

3

R

×

×

×

×

π

=

which is typically less than 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

of the crossover frequency provides sufficient

phase margin. Determine the C3 value by the

following equation:

C

f

3

R

2

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 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 boost 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

MP2358 regulator. The bootstrap diode can be

a low cost one such as IN4148 or BAT54.

MP2358

SW

BS

10nF

5V

MP2358_F02

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty

cycle operation (when

IN

OUT

V

V

>65%) and high