MPQ4458 –INDUSTRIAL GRADE,1A, 4MHz, 36V STEP-DOWN CONVERTER

MPQ4458 Rev. 1.0

www.MonolithicPower.com

12

12/5/2012

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© 2012 MPS. All Rights Reserved.

Compensation Components

MPQ4458 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 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

×

×

×

=

two poles of importance. One is due to the

compensation

capacitor

(C3),

the

output

resistor of error amplifier. 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

=

and

LOAD

2

P

R

2

C

2

1

f

×

×

π

=

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 (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 unstable. A good rule of thumb is to set

the crossover frequency to approximately one-

tenth of the switching frequency or lower. The

Table 4 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 4—Compensation Values for Typical

Output Voltage/Capacitor Combinations

L

R3

C3

C6

1.8V

4.7µH

47µF

ceramic

105k

100pF

None

2.5V

4.7µH-

6.8µH

22µF

ceramic

54.9k

220pF

None

3.3V

6.8µH-

10µH

22µF

ceramic

68.1k

220pF

None

5V

15µH-

22µH

22µF

ceramic

100k

150pF

None

12V

22µH-

33µH

22µF

ceramic

147k

150pF

None

500kHz. Please refer to “Inductor section” on

than that.

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

=

(which typically has a value no higher than