MP2372 – 3A, 28V, 925KHZ STEP-DOWN CONVERTER

MP2372 Rev. 0.92

www.MonolithicPower.com

9

12/22/2009

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due to the output capacitor and the load resistor.

These poles are located at:

VEA

EA

1

P

A

3

C

2

G

f

×

×

π

=

LOAD

2

P

R

2

C

2

1

f

×

×

π

=

530μA/V.

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). This zero is located

at:

3

R

3

C

2

1

Z

×

×

π

=

The system may have another zero of importance,

if the output capacitor has a large capacitance

and/or a high ESR value. The zero, due to the

ESR and capacitance of the output capacitor, is

located at:

ESR

ESR

R

2

C

2

1

f

×

×

π

=

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 or lower. The

switching frequency for the MP2372 is 925KHz,

so the desired crossover frequency is equal to

or less than 92.5KHz.

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

(V)

L (µH)

C2 (µF,

Ceramic)

R3

(kΩ)

C3

(nF)

C6

1

1

47

3

10

None

1.2

1

47

5.1

6.8

None

1.8

2.2

47

7.5

3.3

None

2.5

2.2 - 4.7

47

10

4.7

None

3.3

2.2 - 4.7

47

15

5.6

None

5

4.7 – 6.8

2 x 22

20

4.7

None

12

6.8 - 10

2 x 22

44.2

2.2

None

To optimize the compensation components for

conditions not listed in Table 3, 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

crossover frequency provides sufficient phase

margin. Determine the C3 value by the following

equation:

C

f

3

R

2

4

3

C

×

×

π

>

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 925KHz 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

×

=