MP8670 – 6A, 30V, 420KHZ STEP-DOWN WITH SYNCHRONIZABLE GATE DRIVER

MP8670 Rev. 1.3

www.MonolithicPower.com

13

4/4/2011

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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

AO4474

Alpha & Omega

Semiconductor

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.