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.

__________________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 = 150k

Ω. R2 is given by:

R2 = (R1)

For fixed-output operation, tie FB to REF.

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 100m

Ω. 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.

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 =

45m

Ω (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.

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.

-5V/-12V/-15V or Adjustable,

10

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