MP1583 – 3A, 23V, 385KHz STEP-DOWN CONVERTER

MP1583 Rev. 3.1

www.MonolithicPower.com

8

6/20/2011

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The MP1583 can be optimized for a wide range

of capacitance and ESR values.

Compensation Components

The MP1583 employs current mode control for

easy compensation and fast transient response.

The system stability and transient response are

controlled through the COMP pin. COMP 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:

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A

×

×

×

=

The system has two poles of importance. One

is due to the compensation capacitor (C3) and

the output resistor of error amplifier while the

other is due to the output capacitor and the load

resistor. These poles are located at:

VEA

EA

P

A

C

G

f

×

×

=

3

2

1

π

LOAD

P

R

C

f

×

×

=

2

2

1

2

π

Where

is

the

error

amplifier

transconductance.

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). This zero is located

at:

3

3

2

1

1

R

C

f

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

C

f

×

×

=

2

2

1

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

6

2

1

3

R

C

f

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 unity gain) is

important.

Lower crossover frequencies result in slower

line and load transient responses, while higher

crossover frequencies could cause system

instability. A good standard is to set the

crossover frequency to approximately one-tenth

of the switching frequency. The switching

frequency for the MP1583 is 385KHz, so the

desired crossover frequency is around 38KHz.

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

(Please reference Fig. 3 and Fig. 4)

C2

R3

C3

C6

2.5V

22μF

Ceramic

3.9kΩ

5.6nF

None

3.3V

22μF

Ceramic

4.7kΩ

4.7nF

None

5V

22μF

Ceramic

7.5kΩ

4.7nF

None

12V

22μF

Ceramic

16.9kΩ

1.5nF

None

2.5V

560μF Al.

30mΩ ESR

91kΩ

1nF

150pF

3.3V

560μF Al

30mΩ ESR

120kΩ

1nF

120pF

5V

470μF Al.

30mΩ ESR

100kΩ

1nF

120pF

12V

220μF Al.

30mΩ ESR

169kΩ

1nF

39pF