MP2188 – DUAL 3A, 5.5V, 1.2MHz, SYNCHRONOUS STEP-DOWN SWITCHER

MP2188 Rev. 1.01

www.MonolithicPower.com

14

2/2/2016

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

© 2016 MPS. All Rights Reserved.

The input capacitor can be electrolytic, tantalum

or ceramic. When using electrolytic or tantalum

capacitors, use a small high-quality ceramic

capacitor (0.1μF), placed as close to the IC as

possible. When using ceramic capacitors, make

sure that they have enough capacitance to

prevent excessive voltage ripple at input. The

input voltage ripple caused by capacitance can

be estimated by:

)

V

V

1

(

V

V

C

f

I

V

IN

OUT

IN

OUT

IN

S

LOAD

IN

−

×

×

×

=

Δ

Selecting the Output Capacitor

output DC voltage. Use ceramic capacitors.

Low-ESR capacitors keep the output voltage

ripple low. The output voltage ripple can be

estimated by:

)

C

f

8

1

R

(

)

V

V

1

(

L

f

V

V

OUT

S

ESR

IN

OUT

S

OUT

OUT

×

×

+

×

−

×

×

=

Δ

equivalent series resistance of the output

capacitor.

Using 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:

)

V

V

1

(

C

L

f

8

V

V

IN

OUT

OUT

2

S

OUT

OUT

−

×

×

×

×

=

Δ

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated as:

ESR

IN

OUT

S

OUT

OUT

R

)

V

V

1

(

L

f

V

V

×

−

×

×

=

Δ

The characteristics of the output capacitor also

affect the stability of the regulation system.

PCB Recommendation of MP2188

Proper layout of the switching power supplies is

very important, and sometimes critical for

proper operation. For high-frequency switching

converters, poor layout could lead to poor line

or load regulation and stability issues.

The high current paths (GND, IN, and SW)

should be placed very close to the device using

short, direct, and wide traces. The input

capacitor needs to be as close as possible to

the IN and GND pins. The external feedback

resistors should be placed next to the FB pin.

Keep the switching node SW short and away

from the feedback network.

For MP2188 PCB layout, there are two

recommended layouts for selecting. Figure 2

(refer to schematic figure 4) shows symmetric

PCB layout. If the layout is space limited, and

has to be asymmetric, a ferrite bead (The

Impedance

should

>220mΩ@100MHz)

is

suggested to separate the two input of each

channel. Figure 3 (refer to schematic figure 5)

shows asymmetric PCB layout.

Figure 2: symmetric PCB layout

Figure 3: asymmetric PCB layout