Choosing an Inductor

Practical inductor values range from 10µH to 50µH.

The maximum inductor value is not particularly critical.

For highest current at high

inductor should not be so large that the peak current

never reaches the current limit. That is:

L(max)

≤ _______________________________

This is only important if

More important is that the inductor not be so small that the

current rises much faster than the current-limit comparator

can respond. This would be wasteful and reduce effi-

ciency. Calculate the minimum inductor value as follows:

L(min)

≥ _______________________________

Where L is in µH, 0.3µs is an ample time for the com-

Sense Resistor section), and

δ(I) is the allowable per-

centage of overshoot. As an example, Figure 2's circuit

uses a 3A peak current. If we allow a 15% overshoot

and 15V is the maximum input voltage, then L(min) is

16µH. The actual value of L above this limit has minimal

effect on this circuit's operation.

For highest efficiency, use a coil with low DC resistance.

Coils with 30m

Ω or lower resistance are available. To

minimize radiated noise, use a torroid, pot-core, or shield-

ed-bobbin inductor. Inductors with a ferrite core or equiv-

alent are recommended. Make sure that the inductor’s

Diode Selection

The ICs’ high switching frequencies demand a high-

speed rectifier. Schottky diodes such as the 1N5817 to

1N5822 families are recommended. Choose a diode

with an average current rating approximately equal to

tions, Schottky diodes may be inadequate due to their

high leakage currents; instead, high-speed silicon

diodes may be used. At heavy loads and high temper-

ature, the benefits of a Schottky diode’s low forward

voltage may outweigh the disadvantages of its high

leakage current.

Current-Sense Resistor

The current-sense resistor limits the peak switch cur-

the current-sense resistor, and 210mV is the current-

sense comparator threshold (see Current-Limit Trip

Level in the

Electrical Characteristics).

To maximize efficiency and reduce the size and cost of

external components, minimize the peak current.

However, since the output current is a function of the

peak current, do not set the limit too low. Refer to

Figures 6–9 to determine the sense resistor (and, there-

fore, peak current) for the required load current.

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

12

______________________________________________________________________________________

INPUT VOLTAGE (V)

0

500

1000

1500

2000

2500

34

5 6

7

8

9 10 11 12 13 14 15

Figure 6. MAX774 Maximum Output Current vs. Input Voltage

Figure 7. MAX775 Maximum Output Current vs. Input Voltage

0

200

400

600

800

1000

9

INPUT VOLTAGE (V)

3

4

56

78