LT1073
7
APPLICATIONS INFORMATION
saturation current ratings in the 300mA to 1A range (de-
pending on application) are adequate. Lastly, the inductor
must have sufficiently low DC resistance so that excessive
power is not lost as heat in the windings. An additional
consideration is electro-magnetic interference (EMI).
Toroid and pot core type inductors are recommended in
applications where EMI must be kept to a minimum; for
example, where there are sensitive analog circuitry or
transducers nearby. Rod core types are a less expensive
choice where EMI is not a problem.
Specifying a proper inductor for an application requires
first establishing minimum and maximum input voltage,
output voltage and output current. In a step-up converter,
the inductive events add to the input voltage to produce
the output voltage. Power required from the inductor is
determined by:
P
L
= (V
OUT
+ V
D
– V
IN
)(I
OUT
)
where V
D
is the diode drop (0.5V for a 1N5818 Schottky).
Maximum power in the inductor is
L
L
OSC
=
1
L i
PEAK
2
• f
OSC
where
i
PEAK
=
V
IN
R
1– e
–Rt
ON
L
R = Switch equivalent resistance (1Ω maximum)
added to the DC resistance of the inductor and t
ON
= ON
time of the switch.
At maximum V
IN
and ON time, i
PEAK
should not be al-
lowed to exceed the maximum switch current shown in
Figure 2. Some input/output voltage combinations will
cause continuous
1
mode operation. In these cases a
resistor is needed between I
LIM
(Pin 1) and V
IN
(Pin 2)
to keep switch current under control. See the “Using the
I
LIM
Pin” section for details.
Capacitor Selection
Selecting the right output capacitor is almost as important
as selecting the right inductor. A poor choice for a filter
capacitor can result in poor efficiency and/or high output
ripple. Ordinary aluminum electrolytics, while inexpensive
and readily available, may have unacceptably poor equiva-
lent series resistance (ESR) and ESL (inductance). There
are low-ESR aluminum capacitors on the market specifically
designed for switch-mode DC/DC converters which work
much better than general purpose units. T
antalum capaci-
tors provide still better performance at more expense. We
recommend OS-CON capacitors from Sanyo Corporation
(San Diego, CA). These units are physically quite small and
have extremely low ESR. To illustrate, Figures 3, 4, and 5
show the output voltage of an LT1073 based converter with
three 100µF capacitors. The peak switch current is 500mA
in all cases. Figure 3 shows a Sprague 501D aluminum
capacitor. V
OUT
jumps by over 150mV when the switch
turns off, followed by a drop in voltage as the inductor
dumps into the capacitor. This works out to be an ESR of
over 300mΩ. Figure 4 shows the same circuit, but with a
Sprague 150D tantalum capacitor replacing the aluminum
unit. Output jump is now about 30mV, corresponding to
an ESR of 60mΩ. Figure 5 shows the circuit with an OS-
CON unit. ESR is now only 30mΩ.
In very low power applications where every microampere
is important, leakage current of the capacitor must be
considered. The OS-CON units do have leakage cur-
rent in the 5µA to 10µA range. If the load is also in the
Figure 2. Maximum Switch Current vs Input Voltage
NOTE 1: i.e., inductor current does not go to zero when the switch is off.
V
IN
(V)
0
SWITCH
1000
800
600
400
200
0
1 2 3 4
