Rainbow Electronics MAX776 - Manual (Page 14)

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
typically maintains 120mV p-p output ripple when
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:
V OUT x I OUT x ESR I OUT x t OFF(min) ∆
V p-p = _____________________ + _________________
V
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.
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
Inverting DC-DC Controllers
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