MP4459 – 1.5A, 4MHz, 36V STEP-DOWN CONVERTER

MP4459 Rev. 1.02

www.MonolithicPower.com

13

12/24/2013

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In the case of tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the

output ripple can be approximated to:

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

1

L

f

V

ΔV

×

⎟

⎠

⎞

⎜

⎝

⎛ −

×

×

=

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP1593 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP4459 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.