MP2187 – DUAL 3A, 5.5V, 1.2MHz, SYNCHRONOUS STEP-DOWN SWITCHER

MP2187 Rev. 1.0

www.MonolithicPower.com

15

12/12/2014

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© 2014 MPS. All Rights Reserved.

Since the input capacitor absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

The worse case condition occurs at VIN =

2

I

I

LOAD

1

For simplification, choose an input capacitor

whose RMS current rating greater than half of

the maximum load current.

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:

LOAD

OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

Selecting the Output Capacitor

The output capacitor (C2) maintains the output

DC voltage. Use ceramic capacitors. Low-ESR

capacitors keep the output voltage ripple low.

The output voltage ripple can be estimated by:

OUT

OUT

OUT

ESR

S1

IN

S

VV

1

V1

R

fL

V

8 f

C2

⎛⎞

⎛⎞

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:

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

For tantalum or electrolytic capacitors, the ESR

dominates the impedance at the switching

frequency. For simplification, the output ripple

can be approximated as:

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

The characteristics of the output capacitor also

affect the stability of the regulation system.

PCB Recommendation of MP2187

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 MP2187 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