MP9151

– 20V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP9151 Rev. 1.0

www.MonolithicPower.com

14

3/20/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2013 MPS. All Rights Reserved.

The input capacitor 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:

LOAD

OUT

OUT

IN

IN

S

IN

I

V

V

V1

f

C1

V

V

Selecting the 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

S

1

IN

S

VV

1

V

1

R

f

L

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

IN

S1

VV

ΔV

1

V

8 f

L

C2

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

S1

VV

ΔV

1

R

f

L

V

The characteristics of the output capacitor also

affect the stability of the regulation system. The

MP9151 can be optimized for a wide range of

capacitance and ESR values.

External Bootstrap Diode

An external bootstrap diode may enhance the

efficiency

of

the

regulator,

the

applicable

conditions of external BST diode are:

IN

OUT

V

V

>65%

In these cases, an external BST diode is

recommended from the VCC pin to BST pin, as

shown in Figure 6.

Figure 6: Add Optional External Bootstrap Diode

to Enhance Efficiency

The recommended external BST diode is

IN4148, and the BST cap is 0.1~1

μF.

PC Board Layout

This PCB board layout is referring to the

schematic in Figure 7.

Place the high-current paths (GND, IN and SW)

very close to the device with short, direct and

wide traces. The input decoupling capacitor

needs to be placed as close as possible to the

IN

and GND

pins. The VCC

decoupling

capacitor needs to be placed as close as

possible to the VCC pin and multiple VIAs

should be used on both the ground side of the

VCC decoupling capacitor and the GND pins to

connect to the inner and bottom ground plane.

Place the external feedback resistors next to

the FB pin. Keep the switching node SW short

and away from the feedback network.