5V/3.3V/3V or Adjustable, High-Efficiency,

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11

Standard wire-wound and metal-film resistors have an

inductance high enough to degrade performance.

Surface-mount (chip) resistors have very little induc-

tance and are well suited for use as current-sense

resistors. A wire resistor made by IRC works well in

through-hole applications. Because this resistor is a

band of metal shaped as a “U”, its inductance is less

than 10nH (an order of magnitude less than metal film

resistors). Resistance values between 5m

Ω and 0.1Ω

are available (see Table 1).

Inductor Selection

Practical inductor values range from 10µH to 50µH or more.

The circuit operates in discontinuous-conduction mode if:

V+

RR

P-channel FET. To get the full output capability in

discontinuous-conduction mode, choose an inductor

value no larger than:

L(max) = —————————————————

In both the continuous and discontinuous modes, the

lower limit of the inductor is more important. With a

small inductor value, the current rises faster and over-

shoots the desired peak current limit because the cur-

rent-limit comparator cannot respond fast enough. This

reduces efficiency slightly and, more importantly, could

cause the current rating of the external components

to be exceeded. Calculate the minimum inductor value

as follows:

L(min) = ————————————––——

where

∆I is the percentage of inductor-current over-

it takes the comparator to switch. An overshoot of 10%

is usually not a problem. Inductance values above the

minimum work well if the maximum value defined above

is not exceeded. Smaller inductance values cause

higher output ripple because of overshoot. Larger val-

ues tend to produce physically larger coils.

For highest efficiency, use a coil with low DC resis-

minimize radiated noise, use a toroid, pot core, or

shielded-bobbin inductor. Inductors with a ferrite core

or equivalent are recommended. Make sure the induc-

However, it is generally acceptable to bias the inductor

into saturation by about 20% (the point where the

inductance is 20% below its nominal value).

The peak current of Figure 1 is 2.35A for a 1.5A output.

The inductor used in this circuit is specified to drop by

10% at 2.2A (worst case); a curve provided by the

manufacturer shows that the inductance typically drops

by 20% at 3.1A. Using a slightly underrated inductor

can sometimes reduce size and cost, with only a minor

impact on efficiency. The MAX649/MAX651/MAX652

current limit prevents any damage from an underrated

inductor’s low inductance at high currents.

Table 1 lists inductor types and suppliers for various

applications. The efficiencies of the listed surface-

mount inductors are nearly equivalent to those of the

larger size through-hole versions.

Diode Selection

The MAX649/MAX651/MAX652’s high switching fre-

quency demands a high-speed rectifier (commonly

called a catch diode when used in switching-regulator

circuits). Schottky diodes, such as the 1N5817 through

1N5822 families (and their surface-mount equivalents),

are recommended. Choose a diode with an average

voltage rating higher than V+(max). For high-tempera-

ture applications, where Schottky diodes can be

inadequate because of high leakage currents, use

high-speed silicon diodes instead. At heavy loads and

high temperatures, the disadvantages of a Schottky

diode’s high leakage current may outweigh the benefits

of its low forward voltage. Table 1 lists diode types and

suppliers for various applications.

External Switching Transistor

The MAX649/MAX651/MAX652 drive P-channel

enhancement-mode MOSFET transistors only. The

choice of power transistor is primarily dictated by the

input voltage and the peak current. The transistor's

on-resistance, gate-source threshold, and gate

capacitance must also be appropriately chosen. The

drain-to-source and gate-to-source breakdown voltage

ratings must be greater than V+. The total gate-charge

specification is normally not critical, but values should

be less than 100nC for best efficiency. The MOSFET

should be capable of handling the peak current and,

for maximum efficiency, have a very low on-resistance

at that current. Also, the on-resistance must be low for

Select a transistor with an on-resistance between 50%

and 100% of the current-sense resistor. The Si9430

transistor chosen for the

Typical Operating Circuit has