MP2377 – 3A, 21V, 600KHz STEP-DOWN CONVERTER

MP2377 Rev. 0.92

www.MonolithicPower.com

8

4/19/2011

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APPLICATION INFORMATION

The schematic on the front page shows a typical

MP2377 application. The IC can provide up to

3A output current at a nominal output voltage of

3.3V. For proper thermal performance, the

exposed pad of the device must be soldered

down to the printed circuit board.

Setting the 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). Choose

R1 to be around 40.2kΩ for optimal transient

response. R2 is then given by:

1

V

8

.

0

V

1

R

2

R

=

Table 1—Resistor Selection for Common

Output Voltages

R1 (kΩ)

R2 (kΩ)

1.8

40.2 (1%)

32.4 (1%)

2.5

40.2 (1%)

19.1 (1%)

3.3

40.2 (1%)

13 (1%)

5

40.2 (1%)

7.68 (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 DC resistance should be less than

15mΩ. For most designs, the inductance value

can be derived from the following equation.

OSC

L

IN

OUT

IN

OUT

f

I

V

)

V

V

(

V

L

×

∆

×

−

×

=

Choose inductor current to be approximately

30% of the maximum load current, 3A. The

maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

∆

+

=

Under light load conditions below 100mA, larger

inductance is recommended for improved efficiency.

Output Rectifier Diode

The output rectifier diode supplies the current to

the inductor when the high-side switch is off. To

reduce losses due to the diode forward voltage

and recovery times, use a Schottky diode.

Choose a diode whose maximum reverse

voltage rating is greater than the maximum

input voltage, and whose current rating is

greater than the maximum load current. Table 2

lists

example

Schottky

diodes

and

manufacturers.

Table 2—Diode Selection Guide

Diode

Voltage/Current

Rating

Manufacture

B330

30V, 3A

Diodes Inc.

MBRS330

30V, 3A

On Semi

Selecting the Input Capacitor

The input capacitor (C1) 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 to the input. Ceramic capacitors with

X5R

or

X7R

dielectrics

are

highly

recommended because of their low ESR and

small temperature coefficients. For 3A output

applications, a 10µF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor (C2) keeps output voltage

small and ensures regulation loop stability. The

output capacitor impedance should be low at

the switching frequency. Ceramic capacitors

with X5R or X7R dielectrics are recommended.