MPQ4560

– 2A, 2MHz, 55V, STEP-DOWN CONVERTER

MPQ4560 Rev. 1.1

www.MonolithicPower.com

14

3/29/2013

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© 2013 MPS. All Rights Reserved.

Input Capacitor

The input current to the step-down converter is

discontinuous and requires a capacitor to supply

the AC current to the step-down converter while

maintaining the DC input voltage. Use capacitors

with low equivalent series resistances (ESR) for

the best performance. Ceramic capacitors are

best,

but

tantalum

or

low-ESR

electrolytic

capacitors may also suffice.

For simplification, choose the input capacitor with

an RMS current rating greater than half of the

maximum load current. The input capacitor (C1)

can be electrolytic, tantalum, or ceramic.

When using electrolytic or tantalum capacitors,

place a small, high-quality, ceramic capacitor

(

0.1μF) as close to the IC as possible. When

using ceramic capacitors, make sure that they

have enough capacitance to provide sufficient

charge to prevent excessive voltage ripple at the

input.

The

input

voltage

ripple

caused

by

capacitance is approximately:

























IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

Output Capacitor

The output capacitor (C2) maintains the DC

output voltage. Use ceramic, tantalum, or low-

ESR electrolytic capacitors. Low-ESR capacitors

are preferred to keep the output voltage ripple

low. The output voltage ripple can be estimated

as:











































2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

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 by:

























IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

ΔV

For tantalum or electrolytic capacitors, the ESR

dominates

the

impedance

at

the

switching

frequency. For simplification, the output ripple is

approximately:

ESR

IN

OUT

S

OUT

OUT

R

V

V

1

L

f

V

ΔV























The characteristics of the output capacitor also

affect the stability of the regulation system. The

MPQ4560 can be optimized for a wide range of

capacitances and ESR values.

Compensation Components

MPQ4560 employs current-mode control for easy

compensation and fast transient response. The

COMP pin controls the system stability and

transient response. The COMP pin is the output

of the internal error amplifier. A series capacitor-

resistor

combination

sets

a

pole-zero

combination to control the

control system’s

characteristics. The DC gain of the voltage

feedback loop is:

OUT

FB

VEA

CS

LOAD

VDC

V

V

A

G

R

A









Where

 A

VEA

is the error-amplifier

voltage gain,

400V/V;

 G

5.6A/V; and

 R

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 are located at:

EA

P1

VEA

G

f

2

π C3 A





P2

LOAD

1

f

2

π C2 R





Where,

is

the

error-amplifier

transconductance, 120

μA/V.

The system has one important zero due to the

compensation capacitor and the compensation

resistor (R3). This zero is located at:

Z1

1

f

2

π C3 R3



