MAX775MJA(2002) データシートの表示（PDF） - Maxim Integrated

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MAX775MJA
(Rev.:2002)

-5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-to-DC Controllers

Maxim Integrated 

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

Low IQ Inverting DC-to-DC Controllers

Table 1. Component Suppliers

SUPPLIER

PHONE

INDUCTORS

Coiltronics

(407) 241-7876

Gowanda

(716) 532-2234

Sumida Japan

81-3-3607-5111

Sumida USA

(708) 956-0666

CAPACITORS

Kemet

(803) 963-6300

Matsuo

(714) 969-2491

Nichicon

(708) 843-7500

Sanyo Japan

81-7-2070-6306

Sanyo USA

(619) 661-6835

Sprague

(603) 224-1961

United Chemi-Con (714) 255-9500

DIODES

Motorola

(800) 521-6274

Nihon USA

81-3-3494-7411

Nihon Japan

(805) 867-2555

POWER MOSFETS

Harris

(407) 724-3729

International Rectifier (310) 322-3331

Siliconix

(408) 988-8000

CURRENT-SENSE RESISTORS

IRC

(704) 264-8861

FAX

(407) 241-9339

(716) 532-2702

81-3-3607-5144

(708) 956-0702

(803) 963-6322

(714) 960-6492

(708) 843-2798

81-7-2070-1174

(619) 661-1055

(603) 224-1430

(714) 255-9400

(602) 952-4190

81-3-3494-7414

(805) 867-2556

(407) 724-3937

(310) 322-3332

(408) 970-3950

(704) 264-8866

Capacitors

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

to be consistent with size, ripple, and output voltage

requirements. Place capacitors in parallel if the size

desired is unobtainable. This 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 0.07Ω ESR typically maintains 120mVP-P

output ripple when generating -5V at 1A from a 5V

input. Smaller capacitors 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:

∆VP-P

=

_V_O_U__T__x___IO__U_T___x_E__S_R_

VIN

+

_I_O_U__T_x__t_O_F_F__(m__in_)_

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.

14 ______________________________________________________________________________________

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