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

MP2131 Rev.1.0

www.MonolithicPower.com

13

5/6/2015

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

© 2015 MPS. All Rights Reserved.

APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider is used to set the

output voltage (see Typical Application on page

1). The feedback resistor R1 cannot have too

large or too small a value considering the trade-

off between a dynamic circuit and stability in the

circuit. Choose R1 around 50 kΩ to 200 kΩ.

Choose a larger resistance to get lower leakage

or a smaller resistance to avoid noise. R2 is then

given using Equation (2):

out

R1

R2

V

1

0.6

=

−

(2)

The feedback circuit is shown in Figure 4.

Figure 4—Feedback network

Table 1 lists the recommended resistor values for

common output voltages.

Table 1—Resistor selection for common output

voltages

R1 (kΩ)

R2 (kΩ)

1.0

200(1%)

300(1%)

1.2

200(1%)

200(1%)

1.8

200(1%)

100(1%)

2.5

200(1%)

63.2(1%)

3.3

200(1%)

44.2(1%)

Selecting the Inductor

A 0.82 µH to 2.2 µH inductor is recommended for

most applications. For highest efficiency, the

inductor DC resistance should be less than

15 mΩ. For most designs, the inductance value

is derived from Equation (3).

OUT

IN

OUT

IN

L

S

V(V

V

)

L

VI

f

×−

=

×Δ ×

(3)

the switching frequency.

Choose an inductor current approximately 30

percent of the maximum load current. The

maximum inductor peak current is calculated

using Equation (4)

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

(4)

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low

ESR capacitors for the best performance.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients. For

most applications, a 22 µF capacitor is sufficient.

For a higher output voltage, a 47 µF may be

needed for a more stable system.

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 using Equation (5)

and Equation (6):

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

(5)

where:

2

I

I

LOAD

1

(6)

For simplification, choose an input capacitor with

a 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,

a

small,

high-quality

ceramic

capacitor (i.e., 0.1 μF) should be placed as close

to the IC as possible. When using ceramic

capacitors, make sure 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

using

Equation

(7):