MP8676 – 4.5A, 42V, 420KHZ STEP-DOWN WITH SYNCHRONIZABLE GATE DRIVER

MP8676 Rev.1.0

www.MonolithicPower.com

8

5/7/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

APPLICATION INFORMATION

The schematic on the front page shows a typical

MP8676 application. The IC can provide up to

4.5A 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% 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 ripple current to be approximately

30% of the maximum load current, 4.5A. 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 improving

efficiency.

Synchronous MOSFET

The external synchronous MOSFET is used to

freewheel the inductor current when the internal

high-side switch is off. It significantly reduces

the power loss compared against a Schottky

rectifier.

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 4.5A 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.