-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
DC-DC Inverters
MAX764/MAX765/MAX766
Modes of Operation
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.
In discontinuous-conduction mode, current through the
inductor starts at zero, rises to a peak value, then
ramps down to zero on each cycle. Although efficiency
is still excellent, the output ripple may increase slightly.
Diode Selection
The MAX764/MAX765/MAX766’s high switching fre-
quency demands a high-speed rectifier. Use 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
can use high-speed silicon diodes, such as 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.
Capacitor Selection
Output Filter Capacitor
The primary criterion for selecting the output filter
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 Sanyo OS-CON capacitor with ESR =
45mΩ (SA series) typically provides 50mV ripple when
converting from 5V to -5V at 150mA.
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.
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.
__________________Design Procedure
Setting the Output Voltage
The MAX764/MAX765/MAX766’s output voltage can be
adjusted from -1.0V to -16V using external resistors R1
and R2, configured as shown in Figure 3. For
adjustable-output operation, select feedback resistor
R1 = 150kΩ. R2 is given by:
V
OUT
R2 = (R1) ———
V
REF
where V
REF
= 1.5V.
For fixed-output operation, tie FB to REF.
I I
Inductor Selection
In both continuous- and discontinuous-conduction
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 highest efficiency, use a coil with low DC resis-
tance, preferably under 100mΩ. To minimize radiated
noise, use a toroid, pot core, or shielded coil.
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).
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.
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
action of the MAX764–MAX766. The input voltage
source impedance determines the size of the capacitor
required at the V+ input. As with the output filter
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.
10
______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
