LT1073

7

Rev B

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:

PL = (VOUT + VD – VIN)(IOUT)

where VD is the diode drop (0.5V for a 1N5818 Schottky).

Maximum power in the inductor is

PL=EL• fOSC

= 1

2

L iPEAK

2 • f

OSC

where

iPEAK =

VIN

R











 1– e

–RtON

L













R = Switch equivalent resistance (1Ω maximum)

added to the DC resistance of the inductor and tON = ON

time of the switch.

At maximum VIN and ON time, iPEAK should not be al-

lowed to exceed the maximum switch current shown in

Figure 2. Some input/output voltage combinations will

cause continuous1 mode operation. In these cases a

resistor is needed between ILIM (Pin 1) and VIN (Pin 2)

to keep switch current under control. See the “Using the

ILIM 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

arelow-ESRaluminumcapacitorsonthemarketspecifically

designed for switch-mode DC/DC converters which work

much better than general purpose units. Tantalum capaci-

tors provide still better performance at more expense. We

recommend OS-CON capacitors from Sanyo Corporation

(SanDiego,CA).Theseunitsarephysicallyquitesmalland

have extremely low ESR. To illustrate, Figures 3, 4, and 5

showtheoutputvoltageofanLT1073basedconverterwith

three 100µF capacitors. The peak switch current is 500mA

in all cases. Figure 3 shows a Sprague 501D aluminum

capacitor. VOUT 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.

VIN (V)

0

1200

1000

800

600

400

200

0

1

2

3

4

1073 F02

5