Selecting the Inductor (L)

In both CCM and DCM, practical inductor values range

from 10µH to 50µH. 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. The mini-

2.5µs, so select an inductance that allows the current to

er light-load efficiency and minimizing ripple. Hence, cal-

culate the minimum inductance value as:

L

OR

L

The coil’s inductance need not satisfy this criterion

exactly, as the circuit can tolerate a wide range of val-

ues. Larger inductance values tend to produce physical-

ly larger coils and increase the start-up time, but are oth-

erwise acceptable. Smaller inductance values allow the

coil current to ramp up to higher levels before the switch

can turn off, producing higher ripple at light loads. In

general, an 18µH inductor is sufficient for most applica-

≤ 5V). An 18µH inductor is appropriate for

input voltages up to 3.6V, as calculated above. However,

the same 18µH coil can be used with input voltages up

to 5V with only small increases in peak current, as shown

in Figures 4a and 4b.

Inductors with a ferrite core or equivalent are recom-

mended. The inductor’s incremental saturation-current

rating should be greater than the 1A peak current limit. It

is generally acceptable to bias the inductor into satura-

tion by approximately 20% (the point where the induc-

tance is 20% below the nominal value). For highest effi-

ciency, use a coil with low DC resistance, preferably

under 100m

Ω. To minimize radiated noise, use a toroid,

a pot core, or a shielded coil.

Table 1 lists inductor types and suppliers for various

applications. The listed surface-mount inductors’ efficien-

cies are nearly equivalent to those of the larger through-

hole inductors.

Diode Selection

The MAX761/MAX762’s high switching frequency

demands a high-speed rectifier. Use a Schottky diode

with a 1A average current rating, such as a 1N5817. For

high-temperature applications, use a high-speed silicon

diode, such as the MUR105 or the EC11FS1. These

diodes have lower high-temperature leakage than

Schottky diodes (Table 1).

Capacitor Selection

Output Filter Capacitor

The primary criterion for selecting the output filter capac-

itor (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

33µF, 16V Sanyo OS-CON capacitor with 100m

Ω ESR

typically provides 100mV ripple when stepping up from

5V to 12V at 150mA.

Because the output filter capacitor’s ESR affects efficien-

cy, use low-ESR capacitors for best performance. The

smallest low-ESR SMT tantalum capacitors currently

available are the Sprague 595D series. Sanyo OS-CON

organic semiconductor through-hole capacitors and

Nichicon PL series also exhibit very low ESR. Table 1

lists some suppliers of low-ESR capacitors.

Input Bypass Capacitors

The input bypass capacitor, C1, reduces peak currents

drawn from the voltage source, and also reduces noise

at the voltage source caused by the MAX761/MAX762’s

switching action. 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 recommended. For output currents up to

250mA, 33µF (C1) is adequate, although smaller bypass

capacitors may also be acceptable. Bypass the IC sepa-

rately with a 0.1µF ceramic capacitor, C2, placed close

to the V+ and GND pins.

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

10

______________________________________________________________________________________

LX

GND

MAX761

MAX762

SHDN

V+

REF

L1

18µH

LBI

FB

D1

1N5817

C2

C4

C3

R2

8

2

5

4

7

6

3

R2 = R1

(

-1

)

VOUT

VREF

C1 = 33µF

C2 = 0.1µF

C3 = 0.1µF

C4 = 33µF

C1

R1

Figure 5. Bootstrapped Operation with Adjustable Output