MP2144 Rev. 1.03

www.MonolithicPower.com

11

12/20/2012

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APPLICATION INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The external resistor divider sets the output

voltage (see the Typical Application schematic

on page 1). The design of the feedback resistor

R1 must account for both stability and dynamic

response, and thus can not be too large or too

small. Choose an R1 value between 120kΩ and

200kΩ. R2 is then given by:

out

R1

R2

V

1

0.6

=

−

The feedback circuit is shown in Figure 2.

MP2144

R2

R1

VOUT

FB

Figure 2: Feedback Network

Table 1 lists the recommended resistors values

for common output voltages.

Table 1: Resistor Values 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 0.82µH to 4.7µH inductor is recommended for

most applications. For the best efficiency,

chose an inductor with a DC resistance less

than 15mΩ. For most designs, the inductance

value can be derived from the following

equation.

OUT

IN

OUT

1

IN

L

OSC

V(V

V

)

L

VI

f

×−

=

×Δ ×

Choose an inductor current to be approximately

30% of the maximum load current. The

maximum inductor peak current is:

L

L(MAX)

LOAD

I

II

2

Δ

=+

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, and requires a capacitor 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

values and small temperature coefficients. For

most applications, a 10µF capacitor is sufficient.

For higher output voltage, 47uF may be needed

to increase system stability.

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 is 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 the input.