MP2361 – 2A, 23V, 1.4MHz STEP-DOWN CONVERTER

MP2361 Rev. 1.2

www.MonolithicPower.com

7

1/11/2006

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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 system

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 Boost Diode

For 5V input or 5V output applications, 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 MP2361

regulator. The boost diode can be a low cost

one such as IN4148 or BAT54.

MP2361

SW

BS

5

2

10nF

5V

BOOST

DIODE

MP2361_F02

Figure 2—External Boost Diode