MP38872 – 6A, 21V, 600kHz STEP-DOWN WITH SYNCHRONIZABLE GATE DRIVER

MP38872 Rev.1.11

www.MonolithicPower.com

9

9/21/2011

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© 2011 MPS. All Rights Reserved.

APPLICATION INFORMATION

The schematic on the front page shows a typical

MP38872 application. The IC can provide up to

6A 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

×

Δ

×

−

×

=

Choose inductor current to be approximately

30% of the maximum load current, 6A. The

maximum inductor peak current is:

2

I

I

I

L

LOAD

)

MAX

(

L

Δ

+

=

Under light load conditions below 100mA, larger

inductance is recommended for improved

efficiency.

Synchronous MOSFET

The external synchronous MOSFET is used to

supply current to the inductor when the internal

high-side switch is off. It significantly reduces

the power loss when compared against a

Schottky rectifier.

Table 2 lists example synchronous MOSFETs

and manufacturers.

Table 2—Synchronous MOSFET Selection

Guide

Part No.

Manufacture

FDS6670AS

Fairchild

IRF7821

International Rectifier

AM4874

Analog Power

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 6A output

applications, a 22µF 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.

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 switching node SW short and away

from the feedback network. Keep the EN trace

away from the feedback network, and also an

isolation with ground trace between the EN and

FB pins (highly recommended).