-5V/-12V/-15V or Adjus ta ble ,
High-Effic ie nc y, Low I Inve rting DC-DC Controlle rs
Q
typ ic a lly ma inta ins 120mV
outp ut rip p le whe n
p -p
Table 1. Component Suppliers
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.
SUPPLIER
PHONE
FAX
INDUCTORS
Coiltronics
Gowanda
(407) 241-7876
(716) 532-2234
(708) 956-0666
81-3-3607-5111
(407) 241-9339
(716) 532-2702
(708) 956-0702
81-3-3607-5144
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:
Sumida USA
Sumida Japan
CAPACITORS
Kemet
V
x I
x ESR
I x t
(min)
OUT OFF
OUT
OUT
_____________________
_________________
C
∆V
p-p
=
+
V
IN
(803) 963-6300
(714) 969-2491
(708) 843-7500
(619) 661-6835
81-7-2070-6306
(603) 224-1961
(714) 255-9500
(803) 963-6322
(714) 960-6492
(708) 843-2798
(619) 661-1055
81-7-2070-1174
(603) 224-1430
(714) 255-9400
Matsuo
Whe n e va lua ting this e q ua tion, b e s ure to us e the
c a p a c ita nc e va lue a t the s witc hing fre q ue nc y. 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.
Nichicon
Sanyo USA
Sanyo Japan
Sprague
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.
United Chemi-Con
DIODES
Motorola
(800) 521-6274
(805) 867-2555
81-3-3494-7411
(602) 952-4190
(805) 867-2556
81-3-3494-7414
Nihon USA
Nihon Japan
POWER MOSFETS
Harris
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.
45/MAX76
(407) 724-3729
(310) 322-3331
(408) 988-8000
(407) 724-3937
(310) 322-3332
(408) 970-3950
International Rectifier
Siliconix
CURRENT-SENSE RESISTORS
IRC (704) 264-8861
P C La yo u t a n d Gro u n d in g
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.
(704) 264-8866
Ca p a c it o rs
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
s ize you wa nt is unob ta ina b le . You will not only
inc re a s e the c a p a c ita nc e , b ut a ls o d e c re a s e the
If an external resistor divider is used (Figures 3 and 4),
the trace from FB to the resistors must be extremely short.
c a p a c itor’s ESR (a ma jor c ontrib utor of rip p le ).
330µF tantalum output filter capacitor with 0.07Ω ESR
A
14 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
