ADP1109A

–7–

REV. 0

In practice, the inductor value is easily selected using the equa-

tions above. For example, consider a supply that will generate

12 V at 120 mA from a +5 V source. The inductor power re-

quired is, from Equation 1:

On each switching cycle, the inductor must supply:

=

900 mW

120 kHz

= 7.5 µJ

The required inductor power is fairly low in this example, so

the peak current can also be low. Assuming a peak current of

600 mA as a starting point, Equation 4 can be rearranged to

recommend an inductor value:

L

=

V

IN

I

L MAX

()

t

=

5 V

600 mA

5.5

µs = 45.8 µH

Substituting a standard inductor value of 33

µH, with 0.2 Ω dc

resistance, will produce a peak switch current of:

I

5 V

1.0

Ω

1

− e

–1.0

Ω× 5.5 µs

33

µH











 = 768 mA

Once the peak current is known, the inductor energy can be

calculated from Equation 5:

E

1

2

33

µH

The inductor energy of 9.7

µJ is greater than the P

quirement of 7.5

µJ, so the 33 µH inductor will work in this

application. By substituting other inductor values into the same

equations, the optimum inductor value can be selected. When

selecting an inductor, the peak current must not exceed the

maximum switch current of 1.2 A. If the calculated peak current

is greater than 1.2 A, either the input voltage must be increased

or the load current decreased.

Output Voltage Selection

The output voltage is fed back to the ADP1109A via resistors

R1 and R2 (Figure 5). When the voltage at the comparator’s

inverting input falls below 1.25 V, the oscillator turns “on” and

the output voltage begins to rise. The output voltage is therefore

set by the formula:

V

1

+

R2

R1









Resistors R1 and R2 are provided internally on fixed-voltage

versions of the ADP1109A. In this case, a complete dc-dc con-

verter requires only four external components.

Capacitor Selection

For optimum performance, the ADP1109A’s output capacitor

must be carefully selected. Choosing an inappropriate capacitor

can result in low efficiency and/or high output ripple.

Ordinary aluminum electrolytic capacitors are inexpensive, but

often have poor Equivalent Series Resistance (ESR) and Equiva-

lent Series Inductance (ESL). Low ESR aluminum capacitors,

specifically designed for switch mode converter applications, are

also available, and these are a better choice than general purpose

devices. Even better performance can be achieved with tantalum

capacitors, although their cost is higher. Very low values of ESR

can be achieved by using OS-CON capacitors (Sanyo Corpora-

tion, San Diego, CA). These devices are fairly small, available

with tape-and-reel packaging, and have very low ESR.

Diode Selection

In specifying a diode, consideration must be given to speed,

forward voltage drop and reverse leakage current. When the

ADP1109A switch turns off, the diode must turn on rapidly if

high efficiency is to be maintained. Schottky rectifiers, as well as

fast signal diodes such as the 1N4148, are appropriate. The

forward voltage of the diode represents power that is not

Schottky diodes are recommended. Leakage current is especially

important in low current applications, where the leakage can be

a significant percentage of the total quiescent current.

For most circuits, the 1N5818 is a suitable companion to the

µA to 10 µA

leakage, and fast turn-on and turn-off times. A surface mount

version, the MBRS130T3, is also available.

For switch currents of 100 mA or less, a Schottky diode such as

than 1

µA. A similar device, the BAT54, is available in an

SOT-23 package. Even lower leakage, in the 1 nA to 5 nA range,

can be obtained with a 1N4148 signal diode.

General purpose rectifiers, such as the 1N4001, are not suitable

for ADP1109A circuits. These devices, which have turn-on

times of 10

µs or more, are far too slow for switching power

supply applications. Using such a diode “just to get started” will

result in wasted time and effort. Even if an ADP1109A circuit

appears to function with a 1N4001, the resulting performance

will not be indicative of the circuit performance when the cor-

rect diode is used.