Capacitors

Choose the output capacitor (C4 of Figures 2, 3, and 4)

to be consistent with your size, ripple, and output volt-

age requirements. Place capacitors in parallel if the

size you want is unobtainable. You will not only

increase the capacitance, but also decrease the

capacitor’s ESR (a major contributor of ripple). A

330µF tantalum output filter capacitor with 0.07

Ω ESR

generating -5V at 1A from a 5V input. Smaller capaci-

tors are acceptable for lighter loads or in applications

that can tolerate higher output ripple.

The value of C4 is chosen such that it acquires as

small a charge as possible during the switch on-time.

The amount of ripple as a function of capacitance is

give by:

C

When evaluating this equation, be sure to use the

capacitance value at the switching frequency. At

200kHz, the 330µF tantalum capacitor of Figures 2, 3,

or 4 may degrade by a factor of ten, which will signifi-

cantly alter the ripple voltage calculation.

The ESR of both the bypass and filter capacitors also

affects efficiency. Best performance is obtained by

doubling up on the filter capacitors or using low-ESR

capacitors. Capacitors must have a ripple current rat-

ing equal to the peak current.

The smallest low-ESR SMT capacitors currently avail-

able are the Sprague 595D series. Sanyo OS-CON

organic semiconductor through-hole capacitors also

exhibit low ESR and are especially effective at low tem-

peratures. Table 1 lists the phone numbers of these

and other manufacturers.

PC Layout and Grounding

Due to high current levels and fast switching wave-

forms, proper PC board layout is essential.

Use a star

ground configuration; connect the ground lead of the

input bypass capacitor, the output capacitor, the induc-

tor, and the GND pin of the MAX774/MAX775/MAX776

at a common point very close to the device. Addi-

tionally, input capacitor C2 (Figures 3 and 4) should be

placed extremely close to the device.

If an external resistor divider is used (Figures 3 and 4),

the trace from FB to the resistors must be extremely short.

-5V/-12V/-15V or Adjustable,

14

______________________________________________________________________________________

Table 1. Component Suppliers

SUPPLIER

PHONE

FAX

Coiltronics

(407) 241-7876

(407) 241-9339

Gowanda

(716) 532-2234

(716) 532-2702

Sumida USA

(708) 956-0666

(708) 956-0702

Sumida Japan

81-3-3607-5111

81-3-3607-5144

Kemet

(803) 963-6300

(803) 963-6322

Matsuo

(714) 969-2491

(714) 960-6492

Nichicon

(708) 843-7500

(708) 843-2798

Sanyo USA

(619) 661-6835

(619) 661-1055

Sanyo Japan

81-7-2070-6306

81-7-2070-1174

Sprague

(603) 224-1961

(603) 224-1430

United Chemi-Con

(714) 255-9500

(714) 255-9400

Motorola

(800) 521-6274

(602) 952-4190

Nihon USA

(805) 867-2555

(805) 867-2556

Nihon Japan

81-3-3494-7411

81-3-3494-7414

Harris

(407) 724-3729

(407) 724-3937

International Rectifier

(310) 322-3331

(310) 322-3332

Siliconix

(408) 988-8000

(408) 970-3950

IRC

(704) 264-8861

(704) 264-8866

INDUCTORS

CAPACITORS

DIODES

POWER MOSFETS

CURRENT-SENSE RESISTORS