5V/3.3V or Adjustable, High-Efficiency,

Low-Dropout, Step-Down DC-DC Controllers

10

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Table 1. Component Selection Guide

The peak current of Figure 1 is 2.35A for a 1.5A output.

The inductor used in this circuit is specified to drop by

10% at 2.2A (worst case); a curve provided by the

manufacturer shows that the inductance typically drops

by 20% at 2.7A. Using a slightly underrated inductor

can sometimes reduce size and cost, with only a minor

impact on efficiency.

Table 1 lists inductor types and suppliers for various

applications. The efficiencies of the listed surface-

mount inductors are nearly equivalent to those of the

larger size through-hole versions.

Diode Selection

The MAX1649/MAX1651’s high switching frequency

demands a high-speed rectifier. Schottky diodes, such

as the 1N5817 through 1N5823 (and their surface-

mount equivalents), are recommended. Choose a

diode with an average current rating equal to or greater

V+(max).

External Switching Transistor

The MAX1649/MAX1651 drive P-channel enhancement-

mode MOSFET transistors only. The choice of power

transistor is primarily dictated by the input voltage and

the peak current. The transistor’s on-resistance, gate-

source threshold, and gate charge must also be appro-

priately chosen. The drain-to-source and gate-to-

source breakdown voltage ratings must be greater than

V+. The total gate-charge specification is normally not

critical, but values should be less than 100nC for best

efficiency. The MOSFET should be capable of handling

the peak current and, for maximum efficiency, have a

very low on-resistance at that current. Also, the on-

which equals V+(min). Select a transistor with an on-

resistance between 50% and 100% of the current-

sense resistor. The Si9430 transistor chosen for the

Typical Operating Circuit has a drain-to-source rating

of -20V and a typical on-resistance of 0.070

Ω at 2A with

transistors suitable for use with these devices.

Capacitor Selection

Output Filter Capacitor

The primary criterion for selecting the output filter

capacitor is low equivalent series resistance (ESR),

rather than high capacitance. An electrolytic capacitor

with low enough ESR will automatically have high

enough capacitance. The product of the inductor-cur-

rent variation and the output filter capacitor’s ESR

determines the amplitude of the high-frequency ripple

seen on the output voltage. When a 330µF, 10V

Sprague surface-mount capacitor (595D series) with

ESR = 0.15

Ω is used, 40mV of output ripple is typically

observed when stepping down from 10V to 5V at 1A.

The output filter capacitor's ESR also affects efficiency.

Again, low-ESR capacitors perform best. Table 1 lists

some suppliers of low-ESR capacitors.

PRODUCTION

METHOD

INDUCTORS

CAPACITORS

DIODES

CURRENT-SENSE

RESISTORS

MOSFETS

Surface Mount

AVX

TPS series

Sprague

595D series

Motorola

MBRS340T3

Nihon

NSQ series

Dale

WSL Series

IRC

LRC series

Miniature

Through-Hole

Sumida

RCH875-470M (1.3A)

Sanyo

OS-CON series

low-ESR organic

semiconductor

IRC

OAR series

Motorola

Low-Cost

Through-Hole

CoilCraft

PCH-45-473 (3.4A)

Motorola

1N5817 to

1N5823

Motorola

TMOS power MOSFETs

Sumida

CDRH125-470 (1.8A)

CDRH125-220 (2.2A)

CoilCraft

DO3316-473 (1.6A)

DO3340-473 (3.8A)

Siliconix

Little Foot series

Motorola

medium-power

surface-mount products

Nichicon

PL series

low-ESR electrolytics

United Chemi-Con

LXF series