MP2131 – 4 A, 5.5 V, 1.2 MHZ SYNCHRONOUS STEP-DOWN CONVERTER

MP2131 Rev.1.0

www.MonolithicPower.com

14

5/6/2015

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OUT

OUT

IN

IN

SIN

IV

V

V1

fC1

V

V

⎛⎞

Δ=

×

× −

⎜⎟

×

⎝⎠

(7)

Selecting the Output Capacitor

The output capacitor (C2) is required to maintain

the DC output voltage.

Low ESR ceramic capacitors can be used to

keep the output ripple low. Generally, a 22 μF

output ceramic capacitor will suffice. For a higher

output voltage condition, a 47 μF capacitor may

be needed for a more stable system.

When using ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance. The output voltage ripple is caused

mainly by the capacitance. For simplification, the

output voltage ripple can be estimated with

Equation (8):

OUT

OUT

OUT

2

IN

S1

VV

ΔV1

V

8f

L

C2

⎛⎞

=× −

⎜⎟

×× ×

⎝⎠

(8)

When using tantalum or electrolytic capacitors,

the ESR dominates the impedance at the

switching frequency. For simplification, the output

ripple can be approximated with Equation (9)

OUT

OUT

OUT

ESR

IN

S1

VV

ΔV1

R

fL

V

⎛⎞

=× −

×

⎜⎟

×

⎝⎠

(9)

The characteristics of the output capacitor affect

the stability of the regulation system.

PCB Layout Guidelines

Efficient PCB layout is critical for stable operation.

For best results, refer to Figure 5 and follow the

guidelines below:

1. Place the input decoupling capacitor as

close as possible to the IC pins (a 0805

ceramic capacitor is used).

2. Ensure the two ends of the ceramic

capacitor are connected directly to PVIN

(pin 1) and PGND (pin 3).

Figure 5—Recommended PCB layout: Connect

two ends of the input decoupling capacitor close

to pin 1 and pin 3

Design Example

Table 2 shows a design example when ceramic

capacitors are applied.

Table 2—Design example

The detailed application schematic is shown in

Figure 6. The typical performance and waveforms

have been shown in the “Typical Characteristics”

section. For additional device applications, please

refer to the related evaluation board datasheet.

5 V

1.2 V

4 A