MP1584 – 3A, 1.5MHz, 28V STEP-DOWN CONVERTER

MP1584 Rev. 1.0

www.MonolithicPower.com

12

8/8/2011

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2011 MPS. All Rights Reserved.

The input capacitor (C1) can be electrolytic,

tantalum or ceramic. When using electrolytic or

tantalum capacitors, a small, high quality

ceramic capacitor, i.e. 0.1μF, should be placed

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 input. The

input voltage ripple caused by capacitance can

be estimated by:

























IN

OUT

IN

OUT

S

LOAD

IN

V

V

1

V

V

1

C

f

I

V

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:











































2

C

f

8

1

R

V

V

1

L

f

V

V

S

ESR

IN

OUT

S

OUT

OUT

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:

























IN

OUT

2

S

OUT

OUT

V

V

1

2

C

L

f

8

V

ΔV

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

L

f

V

ΔV























The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP1584 can be optimized for a wide range of

capacitance and ESR values.

Compensation Components

MP1584 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









200V/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:

VEA

EA

1

P

A

3

C

2

G

f









LOAD

2

P

R

2

C

2

1

f









Where,

is

the

error

amplifier

transconductance, 60μA/V.

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









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







