MP28373 – 3A, 28V, 1.4MHz STEP-DOWN CONVERTER

MP28373 Rev. 1.0

www.MonolithicPower.com

8

12/12/2007

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© 2007 MPS. All Rights Reserved.

Compensation Components

MP28373 employs current mode control for

easy compensation and fast transient response.

The system stability and transient response are

controlled through the COMP pin. COMP pin is

the output of the internal transconductance

error amplifier. A series capacitor-resistor

combination sets a pole-zero combination to

control the characteristics of the control system.

The DC gain of the voltage feedback loop is

given by:

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

is the load resistor value.

The system has two poles of importance. One is

due to the compensation capacitor (C3) and the

output resistor of error amplifier, and the other is

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 MP28373 is 1.4MHz,

so the desired crossover frequency is equal to

or less than 140KHz.

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

1.8

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