MP1593 – 3A, 28V, 385KHz STEP-DOWN CONVERTER

MP1593 Rev. 1.9

www.MonolithicPower.com

8

9/14/2006

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Compensation Components

The MP1593 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

given by:

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

1

P

A

3

C

2

G

f

×

×

π

=

LOAD

2

P

R

2

C

2

1

f

×

×

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

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 3), 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 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 MP1593 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.