-5 V/-1 2 V/-1 5 V o r Ad ju s t a b le ,
Hig h -Effic ie n c y, Lo w I DC-DC In ve rt e rs
Q
Mo d e s o f Op e ra t io n
Dio d e S e le c t io n
When delivering high output currents, the MAX764/
MAX765/MAX766 operate in continuous-conduction
mode. In this mode, current always flows in the induc-
tor, and the control circuit adjusts the duty-cycle of the
switch on a cycle-by-cycle basis to maintain regulation
without exceeding the switch-current capability. This
provides excellent load-transient response and high
efficiency.
The MAX764/MAX765/MAX766’s high switching fre-
q ue nc y d e ma nd s a hig h-s p e e d re c tifie r. Us e a
Schottky diode with a 0.75A average current rating,
such as the 1N5817 or 1N5818. High leakage currents
may make Schottky diodes inadequate for high-temper-
ature and light-load applications. In these cases you
c a n us e hig h-s p e e d s ilic on d iod e s , s uc h a s the
MUR105 or the EC11FS1. At heavy loads and high
temperatures, the benefits of a Schottky diode’s low for-
ward voltage may outweigh the disadvantages of its
high leakage current.
In discontinuous-conduction mode, current through the
ind uc tor s ta rts a t ze ro, ris e s to a p e a k va lue , the n
ramps down to zero on each cycle. Although efficiency
is still excellent, the output ripple may increase slightly.
Ca p a c it o r S e le c t io n
__________________De s ig n P ro c e d u re
Output Filter Capacitor
The p rima ry c rite rion for s e le c ting the outp ut filte r
capacitor (C4) is low effective series resistance (ESR).
The product of the inductor-current variation and the
output filter capacitor’s ESR determines the amplitude
of the high-frequency ripple seen on the output voltage.
A 68µF, 20V Sa nyo OS-CON c a p a c itor with ESR =
45mΩ (SA series) typically provides 50mV ripple when
converting from 5V to -5V at 150mA.
S e t t in g t h e Ou t p u t Vo lt a g e
The MAX764/MAX765/MAX766’s output voltage can be
adjusted from -1.0V to -16V using external resistors R1
a nd R2, c onfig ure d a s s hown in Fig ure 3. For
adjustable-output operation, select feedback resistor
R1 = 150kΩ. R2 is given by:
V
OUT
R2 = (R1) ———
Output filter capacitor ESR also affects efficiency. To
obtain optimum performance, use a 68µF or larger,
low-ESR capacitor with a voltage rating of at least
20V. The smallest low-ESR surface-mount tantalum
capacitors currently available are from the Sprague
595D series. Sanyo OS-CON series organic semi-
conductors and AVX TPS series tantalum capacitors
also exhibit very low ESR. OS-CON capacitors are
particularly useful at low temperatures. Table 1 lists
some suppliers of low-ESR capacitors.
V
REF
I
I
45/MAX76
where V
= 1.5V.
REF
For fixed-output operation, tie FB to REF.
In d u c t o r S e le c t io n
In b oth c ontinuous - a nd d is c ontinuous -c ond uc tion
modes, practical inductor values range from 22µH to
68µH. If the inductor value is too low, the current in the
coil will ramp up to a high level before the current-limit
comparator can turn off the switch, wasting power and
reducing efficiency. The maximum inductor value is not
critical. A 47µH inductor is ideal for most applications.
For best results when using capacitors other than those
suggested in Table 1 (or their equivalents), increase
the output filter capacitor’s size or use capacitators in
parallel to reduce ESR.
For highest efficiency, use a coil with low DC resis-
tance, preferably under 100mΩ. To minimize radiated
nois e , us e a toroid , p ot c ore , or s hie ld e d c oil.
Inductors with a ferrite core or equivalent are recom-
mended. The inductor’s incremental saturation-current
rating should be greater than the 0.75A peak current
limit. It is generally acceptable to bias the inductor into
saturation by approximately 20% (the point where the
inductance is 20% below the nominal value).
Input Bypass Capacitor
The input bypass capacitor, C1, reduces peak currents
drawn from the voltage source and reduces the amount
of noise at the voltage source caused by the switching
a c tion of the MAX764–MAX766. The inp ut volta g e
source impedance determines the size of the capacitor
re q uire d a t the V+ inp ut. As with the outp ut filte r
capacitor, a low-ESR capacitor is highly recommended.
For output currents up to 250mA, a 100µF to 120µF
capacitor with a voltage rating of at least 20V (C1) in
parallel with a 0.1µF capacitor (C2) is adequate in most
applications. C2 must be placed as close as possi-
ble to the V+ and GND pins.
Table 1 lists inductor types and suppliers for various
applications. The listed surface-mount inductors’ effi-
ciencies are nearly equivalent to those of the larger-
size through-hole inductors.
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
