MP4561 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER

MP4561 Rev. 1.0

www.MonolithicPower.com

12

11/5/2012

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Output Capacitor

The output capacitor (C2) is required to maintain

the DC output voltage. Ceramic, tantalum, or low

ESR electrolytic capacitors are recommended.

Low ESR capacitors are preferred to keep the

output voltage ripple low. The output voltage

ripple can be estimated by:

OUT

OUT

OUT

ESR

SIN

S

VV

1

ΔV1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

=× −

×

+

⎜⎟

⎜⎟

×× ×

⎝⎠ ⎝⎠

equivalent series resistance (ESR) value of the

output capacitor.

In the case of ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated by:

OUT

OUT

OUT

2

SIN

VV

ΔV1

8f

L C2

V

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

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:

OUT

OUT

OUT

ESR

IN

S

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP4561 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP4561 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:

FB

VDC

LOAD

CS

VEA

OUT

V

AR

G

A

V

=×

×

×

400V/V;

is

the

current

sense

resistor value.

The system has 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:

EA

P1

VEA

G

f

2 π C3 A

=

××

P2

LOAD

1

f

2π C2 R

=

××

Where,

is

the

error

amplifier

transconductance, 120μA/V.

The system has one zero of importance, due to

the compensation capacitor (C3) and the

compensation resistor (R3). This zero is located

at:

Z1

1

f

2 π C3 R3

=

××

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

1

f

2π C2 R

=

××

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:

P3

1

f

2 π C6 R3

=

××