LT1073

7

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:

Maximum power in the inductor is

PE

f

Li

f

L

L

OSC

PEAK

OSC

=

=

•

•

1

2

2

where

i

V

R

e

Rt

L

PEAK

IN

ON

=

















1–

–

R = Switch equivalent resistance (1

Ω maximum)

time of the switch.

to exceed the maximum switch current shown in Figure 2.

Some input/output voltage combinations will cause con-

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 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 100

µF capacitors. The peak

switch current is 500mA in all cases. Figure 3 shows a

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 300m

Ω.Figure4showsthe

same circuit, but with a Sprague 150D tantalum capacitor

replacing the aluminum unit. Output jump is now about

30mV, corresponding to an ESR of 60m

Ω. Figure 5 shows

the circuit with an OS-CON unit. ESR is now only 30m

Ω.

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 5

µA to 10µA range. If the load is also in the

APPLICATIO S I FOR ATIO

0

1200

1000

800

600

400

200

0

123

4

1073 F02

5

Figure 2. Maximum Switch Current vs Input Voltage

NOTE 1: i.e., inductor current does not go to zero when the switch is off.