MP2360 – 1.8A, 24V, 1.4MHz STEP-DOWN CONVERTER

MP2360 Rev. 0.9

www.MonolithicPower.com

6

8/21/2006

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© 2006 MPS. All Rights Reserved.

TM

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

×

∆

×

−

×

=

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

∆

+

=

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.

External Bootstrap Diode

It is recommended that an external bootstrap

diode be added when the input voltage is no

greater than 5V or the 5V rail is available in the

system. This helps improve the efficiency of the

regulator. The bootstrap diode can be a low cost

one such as IN4148 or BAT54.

MP2360

SW

BST

2

7

10nF

5V (External) or

Figure 2—External Bootstrap Diode

This diode is also recommended for high duty cycle

operation (when

IN

OUT

V

V

>65%) applications.