Step 3:

inductor of less than 100µH.

Step 4:

the inductor’s maximum current rating,

whichever is lower.

Inductor series resistance affects both efficiency and

dropout voltage. A high series resistance severely limits

the maximum current available at lower input voltages.

Output currents up to 225mA are possible if the induc-

tor has low series resistance. Inductor and series

inductor’s peak current will fall short of the desired

To maximize efficiency, choose the highest-value

inductor that will provide the required output current

over the whole range of your input voltage (see Typical

Operating Characteristics). Inductors with peak cur-

rents in the 600mA range do not need to be very large.

They are about the size of a 1W resistor, with surface-

mount versions less than 5mm in diameter. Table 1 lists

suppliers of inductors suitable for use with the

MAX639/MAX640/MAX653.

Output Filter Capacitor

The MAX639/MAX640/MAX653’s output ripple has two

components. One component results from the variation

in stored charge on the filter capacitor with each LX

pulse. The other is the product of the current into the

capacitor and the capacitor’s equivalent series resis-

tance (ESR).

The amount of charge delivered in each oscillator pulse

is determined by the inductor value and input voltage.

It decreases with larger inductance, but increases as

the input voltage lessens. As a general rule, a smaller

amount of charge delivered in each pulse results in

less output ripple.

With low-cost aluminum electrolytic capacitors, the

ESR-induced ripple can be larger than that caused by

the charge variation. Consequently, high-quality alu-

minum-electrolytic or tantalum filter capacitors will mini-

mize output ripple. Best results at reasonable cost are

typically achieved with an aluminum-electrolytic capac-

itor in the 100µF range, in parallel with a 0.1µF ceramic

capacitor (Table 1).

External Diode

In most MAX639/MAX640/MAX653 circuits, the current

in the external diode (D1, Figure 3) changes abruptly

from zero to its peak value each time LX switches off.

To avoid excessive losses, the diode must have a fast

turn-on time. For low-power circuits with peak currents

less than 100mA, signal diodes such as the 1N4148

perform well. The 1N5817 diode works well for high-

power circuits, or for maximum efficiency at low power.

1N5817 equivalent diodes are also available in surface-

mount packages (Table 1). Although the 1N4001 and

other general-purpose rectifiers are rated for high cur-

rents, they are unacceptable because their slow turn-

off times result in excessive losses.

Minimum Load

Under no-load conditions, because of leakage from the

PMOS power switch (see the LX Leakage Current vs.

Temperature graph in the Typical Operating

Characteristics) and from the internal resistor from V+

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

10

______________________________________________________________________________________

GND

MAX639

MAX640

MAX653

SHDN

6

VOUT

4

LBI

L = 100

µH

3

1

5

100

µF

1N5817

100

µF

VFB 7

R3

R4

8

V+

LX

OUTPUT

INPUT

+4.0V TO +11.5V

Figure 4. Adjustable-Output Operation

MAX639

MAX640

MAX653

Figure 5. Through-Hole PC Layout and Component Placement

Diagram for Standard Step-Down Application (Top-Side View)