MP38874 – 8A, 21V, 600KHz STEP-DOWN WITH SYNCHRONOUS GATE DRIVER

MP38874 Rev. 1.1

www.MonolithicPower.com

9

10/13/2010

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© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

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% if the maximum load current, 8A. 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.

Synchronous MOSFET

The external synchronous MOSFET is used to

supply current to the inductor when the internal

high-side switch is off. It reduces the power loss

significantly when compared against a Schottky

rectifier.

Table 2 lists example synchronous MOSFETs

and manufacturers.

Table 2—Synchronous MOSFET Selection

Guide

Part No.

Manufacture

Si7112

Vishay

Si7114

Vishay

AM4874

Analog Power

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input and also 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

pass to 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 22µF capacitor is sufficient.

Selecting the Output Capacitor

The output capacitor 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 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.