The standard operating circuits use a 22µH inductor.

If a different inductance value is desired, select L

such that:

L

≥ ——————————

Larger inductance values tend to increase the start-up

time slightly, while smaller inductance values allow the

coil current to ramp up to higher levels before the

switch turns off, increasing the ripple at light loads.

Inductors with a ferrite core or equivalent are recom-

mended; powder iron cores are not recommended for

use with high switching frequencies. Make sure the

inductor’s saturation current rating (the current at which

the core begins to saturate and the inductance starts to

However, it is generally acceptable to bias the inductor

into saturation by approximately 20% (the point where

the inductance is 20% below the nominal value). For

highest efficiency, use a coil with low DC resistance,

preferably under 20m

Ω. To minimize radiated noise,

use a toroid, a pot core, or a shielded coil.

Table 2 lists inductor suppliers and specific recom-

mended inductors.

Power Transistor Selection

Use an N-channel MOSFET power transistor with the

MAX770/MAX771/MAX772 (Figure 8a).

Use an N-FET whenever possible with the MAX773. An

NPN transistor can be used, but be extremely careful

when determining the base current (see

NPN

Transistors section). An NPN transistor is not recom-

mended when using the shunt regulator.

N-Channel MOSFETs

To ensure the external N-channel MOSFET (N-FET) is

turned on hard, use logic-level or low-threshold

N-FETs when the input drive voltage is less than 8V. This

applies even in bootstrapped mode, to ensure start-up.

N-FETs provide the highest efficiency because they do

not draw any DC gate-drive current, but they are typi-

cally more expensive than NPN transistors. When using

an N-FET with the MAX773, connect EXTH and EXTL to

the N-FET’s gate (Figure 8b).

When selecting an N-FET, three important parameters

results; the maximum value is usually grossly over-

specified since it is a guaranteed limit and not the mea-

sured value. The typical total gate charge should be

50nC or less. With larger numbers, the EXT pins may

not be able to adequately drive the gate. The EXT

rise/fall time with various capacitive loads as shown in

the

Typical Operating Characteristics.

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

16

______________________________________________________________________________________

MAX770

MAX771

MAX772

N

EXT

CS

MAX773

NPN

EXTH

EXTL

CS

L

Figure 8a. Use an N-Channel MOSFET with the

MAX770/MAX771/MAX772

Figure 8b. Using an N-Channel MOSFET with the MAX773

Figure 8c. Using an NPN Transistor with the MAX773

MAX773

N

EXTH

EXTL

CS

L