5V/3.3V/3V or Adjustable, High-Efficiency,

12

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Table 1. Component Selection Guide

PRODUCTION

METHOD

INDUCTORS

CAPACITORS

DIODES

CURRENT-SENSE

RESISTORS

MOSFETS

Surface Mount

Matsuo

267 series

Sprague

595D series

Nihon

NSQ series

IRC

LRC series

Miniature

Through-Hole

Sumida

RCH855-220M

Sanyo

OS-CON series

low-ESR organic

semiconductor

IRC

OAR series

Motorola

Low-Cost

Through-Hole

Renco

RL 1284-22

Motorola

1N5820,

1N5823

Motorola

TMOS power MOSFETs

Sumida

CDR125-220 (22µH)

Coiltronics

CTX 100 series

Siliconix

Little Foot series

Motorola

medium-power

surface-mount products

Nichicon

PL series

low-ESR electrolytics

United Chemi-Con

LXF series

a drain-to-source rating of -20V and a typical on-resis-

tance of 0.115

list suppliers of switching 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 ESR of the output filter capacitor

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

Ωis 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.

Use low-ESR capacitors for best performance. The

smallest low-ESR SMT tantalum capacitors currently

available are from the Sprague 595D series. Sanyo OS-

CON organic semiconductor through-hole capacitors

and the Nichicon PL series also exhibit very low ESR.

Table 1 lists some suppliers of low-ESR capacitors.

Input Bypass Capacitor

The input bypass capacitor reduces peak currents

drawn from the voltage source, and also reduces the

amount of noise at the voltage source caused by the

switching action of the MAX649/MAX651/MAX652.

The input voltage source impedance determines the

size of the capacitor required at the V+ input. As

with the output filter capacitor, a low-ESR capacitor

is recommended. Bypass the IC separately with a

0.1µF ceramic capacitor placed close to the V+ and

GND pins.

Reference Capacitor

Bypass REF with a 0.1µF or larger capacitor. REF can

source at least 100µA.

Layout Considerations

Proper PC board layout is essential because of high

current levels and fast switching waveforms that radiate

noise. Minimize ground noise by connecting the anode

of the catch diode, the input bypass capacitor ground

lead, and the output filter capacitor ground lead to a

single point (“star” ground configuration). A ground

plane is recommended. Also minimize lead lengths to

reduce stray capacitance, trace resistance, and radiat-

ed noise. In particular, the traces connected to FB (if an

external resistor divider is used) and EXT must be

short. Place the 0.1µF ceramic bypass capacitor as

close as possible to V+ and GND.