MP4575

– 5A, 55V, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4575 Rev. 1.0

www.MonolithicPower.com

18

8/16/2016

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© 2016 MPS. All Rights Reserved.























IN

OUT

IN

OUT

SW

LOAD

IN

V

V

1

V

V

C1

f

I

ΔV

(8)

Selecting the Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR

electrolytic

capacitors.

Low

ESR

capacitors are recommended to keep the output

voltage ripple low. The output voltage ripple can

be estimated with Equation (9):









































C2

f

8

1

R

V

V

1

L

f

V

ΔV

SW

ESR

IN

OUT

SW

OUT

OUT

(9)

ESR value of the output capacitor.

For

ceramic

capacitors,

the

capacitance

dominates the impedance at the switching

frequency and contributes the most to the

output voltage ripple. For simplification, the

output voltage ripple can be estimated with

Equation (10):

























IN

OUT

2

SW

OUT

OUT

V

V

1

C2

L

f

8

V

ΔV

(10)

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated with Equation (11):

ESR

IN

OUT

SW

OUT

OUT

R

V

V

1

L

f

V

ΔV













 







(11)

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP4575 can be optimized for a wide range of

capacitances and ESR values.

Compensation Components

The MP4575 employs current-mode control for

easy compensation and fast transient response.

COMP is the output of the internal error

amplifier and controls system stability and

transient response. A series resistor-capacitor

combination sets a pole-zero combination to

control the control system’s characteristics. The

DC gain of the voltage feedback loop can be

calculated with Equation (12):

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A









(12)

(1000V/V),

is

the

current-sense

load resistor value.

The system has two important poles: one from

the compensation capacitor (C3) and the output

resistor of error amplifier, and the other due to

the output capacitor and the load resistor.

These poles can be determined with Equation

(13) and Equation (14):

VEA

EA

P1

A

C3

2

π

G

f







(13)

LOAD

P2

R

C2

2

π

1

f







(14)

Where

is

the

error-amplifier

transconductance (540

μA/V).

The system has one important zero due to the

compensation capacitor and the compensation

resistor (R3). This zero can be determined with

Equation (15):

R3

C3

2

π

1

f

Z1







(15)

The system may have another significant zero if

the output capacitor has a large capacitance or

a high ESR value. This zero can be determined

with Equation (16):

ESR

ESR

R

C2

2

π

1

f







(16)

In

this

case,

a

third

pole

set

by

the

compensation

capacitor

(C4)

and

the

compensation resistor can compensate for the

effect of the ESR zero. This pole can be

determined with Equation (17):

R3

C4

2

π

1

f

P3







(17)

The goal of compensation design is to shape

the converter transfer function for a desired

loop gain. The system crossover frequency

where the feedback loop has unity gain is

important, since lower crossover frequencies

result

in

slower

line

and

load

transient

responses, while higher crossover frequencies

lead to system instability. Generally, set the