LT1073_1 Datasheet, PDF (7/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1073
APPLICATIO S I FOR ATIO
saturation current ratings in the 300mA to 1A range
(depending on application) are adequate. Lastly, the in-
ductor 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 recom-
mended 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 deter-
mined 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
P_L= E_L• f_OSC
=
1L
2
i_PEAK2
•
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 allowed
to exceed the maximum switch current shown in Figure 2.
Some input/output voltage combinations will cause con-
tinuous¹ 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.
NOTE 1: i.e., inductor current does not go to zero when the switch is off.
1200
1000
800
600
400
200
0
0
1
2
3
4
5
V_IN (V)
1073 F02
Figure 2. Maximum Switch Current vs Input Voltage
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 specifi-
cally designed for switch-mode DC/DC converters which
work much better than general purpose units. Tantalum
capacitors provide still better performance at more ex-
pense. 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 current in
the 5µA to 10µA range. If the load is also in the
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