9

LT1111

1111fd

specifically designed for switch mode DC/DC converters

which work much better than general-purpose units.

Tantalum capacitors 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 1, 2, and 3 show the output voltage of an

LT1111 based converter with three 100

µF capacitors. The

peak switch current is 500mA in all cases. Figure 1 shows

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 0.24

Ω. Figure 2 shows the

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 0.07

Ω.

Figure 3 shows the circuit with a 16V OS-CON unit. ESR

is now only 0.02

Ω.

S

APPLICATI

I FOR ATIO

switch closes, current in the inductor builds according to

It

V

R

e

L

L

Rt

L

′

















′

115

–( )

–

where R

As an example, suppose –5V at 50mA is to be generated

from a 4.5V to 5.5V input. Recalling Equation (14),

(16)

Energy required from the inductor is:

P

f

mW

kHz

J

L

OSC

==

275

72

38

17

..

( )

µ

Picking an inductor value of 56

µH with 0.2Ω DCR results

in a peak switch current of:

I

VV

emA

PEAK

s

H

+

()















 =

×

45

075

065

0 2

1

445

085

7

56

.– .

..

–.

–.

ΩΩ

Ωµ

µ

EH

A

J

()(

1

2

56

0 445

5 54

19

2

µµ

..

.

( )

Since 5.54

µJ > 3.82µJ, the 56µH inductor will work.

the step-down case, peak switch current should be limited

to ~650mA.

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

equivalent series resistance (ESR) and ESL (inductance).

There are low ESR aluminum capacitors on the market

5

µs/DIV

LT1111 • F01

Figure 1. Aluminum

5

µs/DIV

LT1111 • F02

Figure 2. Tantalum

5

µs/DIV

LT1111 • F01

Figure 3. OS-CON

(18)