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MAX774CPA View Datasheet(PDF) - Maxim Integrated

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MAX774CPA Datasheet PDF : 16 Pages
First Prev 11 12 13 14 15 16
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low IQ Inverting DC-DC Controllers
Table 1. Component Suppliers
SUPPLIER
PHONE
INDUCTORS
Coiltronics
(407) 241-7876
Gowanda
(716) 532-2234
Sumida USA
(708) 956-0666
Sumida Japan
81-3-3607-5111
CAPACITORS
Kemet
(803) 963-6300
Matsuo
(714) 969-2491
Nichicon
(708) 843-7500
Sanyo USA
(619) 661-6835
Sanyo Japan
81-7-2070-6306
Sprague
(603) 224-1961
United Chemi-Con (714) 255-9500
DIODES
Motorola
(800) 521-6274
Nihon USA
(805) 867-2555
Nihon Japan
81-3-3494-7411
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
(708) 956-0702
81-3-3607-5144
(803) 963-6322
(714) 960-6492
(708) 843-2798
(619) 661-1055
81-7-2070-1174
(603) 224-1430
(714) 255-9400
(602) 952-4190
(805) 867-2556
81-3-3494-7414
(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 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 0.07ESR
typically maintains 120mVp-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:
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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