MP2270 – 1.8A, 24V, 700KHz STEP-DOWN CONVERTER

MP2270 Rev. 1.1

www.MonolithicPower.com

8

4/16/2010

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APPLICATION INFORMATION

Setting Output Voltage

The external resistor divider is used to set the

output voltage (see the schematic on front

page). The feedback resistor R1 also sets the

feedback loop bandwidth with the internal

compensation capacitor (see Figure 1). R2 can

be determined by:

1

V

81

.

0

V

1

R

2

R

=

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

80.6 (1%)

64.9 (1%)

2.5

49.9 (1%)

23.7 (1%)

3.3

49.9 (1%)

16.2 (1%)

5

49.9 (1%)

9.53 (1%)

Selecting the Inductor

A 1µH to 10µH inductor with a DC current rating

of at least 25% percent higher than the

maximum load current is recommended for

most applications. For highest efficiency, the

inductor’s DC resistance should be less than

200mΩ. For most designs, the required

inductance value can be derived from the

following equation.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

Δ

×

−

×

=

Choose an inductor with a rating current of

approximately 30% higher than the maximum

load current if the maximum load current is

≥1.8A. The maximum inductor peak current is

calculated from:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, a

larger inductance is recommended for improved

efficiency. See Table 2 for suggested inductors.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input and the switching noise from

the device. The input capacitor impedance at the

switching frequency should be less than the input

source impedance to prevent high frequency

switching current from passing through the input.

Ceramic capacitors with X5R or X7R dielectrics

are highly recommended because of their low

ESR and small temperature coefficients. For most

applications, a 10µF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor keeps output voltage ripple

small and ensures loop stability. The output

capacitor impedance should be low at the

switching frequency. Ceramic capacitors with

X5R or X7R dielectrics are recommended for their

low ESR characteristics. For most applications, a

22µF ceramic capacitor will be sufficient.

PC Board Layout

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

be placed very close to the device with 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 switch node traces

short and away from the feedback network.