LT1173

7

As an example, suppose 9V at 50mA is to be generated

from a 3V input. Recalling Equation 02,

(07)

Energy required from the inductor is

P

F

mW

kHz

J

L

OSC

==

()

325

24

13 5

08

..

µ

Picking an inductor value of 100

µH with 0.2Ω DCR results

in a peak switch current of

i

V

em A

PEAK

s

H

=











 =

()

•

3

1

1

616

09

123

100

Ω

Ω

–.

–

µ

µ

EH

A

J

()(

()

1

2

100

0 616

19 0

10

2

µµ

..

.

Since 19

µJ > 13.5µJ the 100µH inductor will work. This

trial-and-error approach can be used to select the opti-

mum inductor. Keep in mind the switch current maximum

rating of 1.5A. If the calculated peak current exceeds this,

consider using the LT1073. The 70% duty cycle of the

LT1073 allows more energy per cycle to be stored in the

inductor, resulting in more output power.

An inductor’s energy storage capability is proportional to

its physical size. If the size of the inductor is too large for

a particular application, considerable size reduction is

possible by using the LT1111. This device is pin compat-

ible with the LT1173 but has a 72kHz oscillator, thereby

reducing inductor and capacitor size requirements by a

factor of three.

For both positive-to-negative (Figure 7) and negative-to-

positive configurations (Figure 8), all the output power

must be generated by the inductor. In these cases

(11)

In the positive-to-negative case, switch drop can be mod-

eled as a 0.75V voltage source in series with a 0.65

Ω

resistor so that

(12)

In the negative-to-positive case, the switch saturates and

the 0.8

Ω switch ON resistance value given for Equation 04

can be used. In both cases inductor design proceeds from

Equation 03.

The step-down case is different than the preceeding three

in that the inductor current flows through the load in a

step-down topology (Figure 6). Current through the switch

should be limited to ~650mA in step-down mode. This can

in the range of 12V to 25V, a 5V output at 300mA can be

generated with a 220

µH inductor and 100Ω resistor in

470

µH inductor should be used along with the 100Ω

resistor.

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 2, 3, and 4 show the output voltage of an LT1173

based converter with three 100

µF capacitors. The peak

switch current is 500mA in all cases. Figure 2 shows a

over 120mV 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 240m

Ω.Figure3showsthe

same circuit, but with a Sprague 150D, 20V tantalum

capacitor replacing the aluminum unit. Output jump is

now about 35mV, corresponding to an ESR of 70m

Ω.

Figure 4 shows the circuit with a 16V OS-CON unit. ESR is

now only 20m

Ω.

S

APPLICATI

I FOR ATIO