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:
P
L
= (12 V + 0.5 V – 5 V) × (120 mA) = 900 mW
On each switching cycle, the inductor must supply:
P
L
f
OSC
=
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=
5V
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
PEAK
=
5V
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
L
=
1
2
33 µH
()
×768 mA
()
2
=9.7 µJ
The inductor energy of 9.7 µJ is greater than the P
L
/f
OSC
re-
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
OUT
= 1. 25 V × 1 +
R 2
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
delivered to the load, so V
F
must also be minimized. Again,
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
ADP1109A. This diode has a V
F
of 0.5 V at 1 A, 4 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
the BAT85 provides a V
F
of 0.8 V at 100 mA and leakage less
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.
