MP38671 – 10A, 25V, 600KHz STEP-DOWN WITH SYNCHRONOUS GATE DRIVER

MP38671 Rev. 1.1

www.MonolithicPower.com

9

9/21/2011

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© 2011 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)

1.2

40.2 (1%)

80.6 (1%)

1.8

40.2 (1%)

32.4 (1%)

2.5

40.2 (1%)

19.1 (1%)

Selecting the Inductor

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

of at least 25% higher than the maximum load

current is recommended for most applications.

For

highest

efficiency,

the

inductor

DC

resistance should be less than 10mΩ. For most

designs, the inductance value can be derived

from the following equation.

OUT

IN

OUT

IN

L

OSC

V(V

V

)

L1

VI

f

×−

=

×Δ ×

Choose

inductor

ripple

current

to

be

approximately 30% of the maximum load

current, 10A. 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 improving

efficiency.

Synchronous MOSFET

The external synchronous MOSFET is used to

freewheel the inductor current when the internal

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

significantly compared against a Schottky

rectifier.

Table 2 lists example synchronous MOSFETs

and manufacturers.

Table 2—Synchronous MOSFET Selection

Guide

Part No.

Manufacture

Si7114

Vishay

HAT2165

Renesas

Selecting the Input Capacitor

The input current to the step-down converter is

discontinuous, therefore a capacitor is required to

supply the AC current to the step-down converter

while maintaining the DC input voltage. Use low

ESR capacitors for the best performance. 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.

Since the input capacitor (C1) absorbs the input

switching current it requires an adequate ripple

current rating. The RMS current in the input

capacitor can be estimated by:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

−

×

=

IN

OUT

IN

OUT

LOAD

1

C

V

V

1

V

V

I

I

where:

2

I

I

LOAD

1

For simplification, choose the input capacitor

whose RMS current rating greater than half of

the maximum load current.