ADP1108

–7–

REV. 0

where: DC = duty cycle (0.7 for the ADP1108)

As previously mentioned, the switch voltage is higher in step-

The inductor value can now be calculated:

L

=

×t

ON

(Equation 7)

µs)

If the input voltage will vary (such as an application that must

switch current constant as the input voltage rises. Note that

of operation.

For example, assume that +5 V at 250 mA is required from a +9 V

to +18 V source. Deriving the peak current from Equation 6

yields:

I

2

× 250 mA

0.7

5

+ 0.5

9

− 1.5 + 0.5









= 491 mA

The peak current can than be inserted into Equation 7 to cal-

culate the inductor value:

L

= 9–1.5–5

491 mA

× 36 µs = 183 µH

Since 183

µH is not a standard value, the next lower standard

value of 150

µH would be specified.

To avoid exceeding the maximum switch current when the in-

ing Figure 4, a value of 160

Ω will limit the switch current to

500 mA.

Inductor Selection—Positive-to-Negative Converter

The configuration for a positive-to-negative converter using the

ADP1108 is shown in Figure 19. As with the step-up converter,

all of the output power for the inverting circuit must be supplied

by the inductor. The required inductor power is derived from

the formula:

P

OUT

()

(Equation 8)

The ADP1108 power switch does not saturate in positive-to-

negative mode. The voltage drop across the switch can be mod-

eled as a 0.75 V base-emitter diode in series with a 0.65

Ω

resistor. When the switch turns on, inductor current will rise at

a rate determined by:

I

V

L

R'

1

− e

– R't

L













(Equation 9)

where: R' = 0.65

Ω + R

For example, assume that a –5 V output at 100 mA is to be gen-

erated from a +4.5 V to +5.5 V source. The power in the induc-

tor is calculated from Equation 8:

P

During each switching cycle, the inductor must supply the fol-

lowing energy:

=

550 mW

19 kHz

= 28.9µ J

Using a standard inductor value of 220

µH with 0.3 Ω dc resis-

tance will produce a peak switch current of:

I

4.5V –0.75 V

0.65

Ω+ 0.3 Ω

1

− e

–0.95

Ω× 36 µs

220

µH











 = 568 mA

Once the peak current is known, the inductor energy can be cal-

culated from Equation 9:

1

2

220

µH × 568 mA







 = 35.5µ J

The inductor energy of 35.5

µJ is greater than the P

quirement of 28.9

µJ, so the 220 µH inductor will work in

this application.

To avoid exceeding the maximum switch current when the in-

Referring to Figure 4, a value of 150

is appropriate in this

application.

Capacitor Selection

For optimum performance, the ADP1108’s output capacitor

must be carefully selected. Choosing an inappropriate capacitor

can result in low efficiency and/or high output ripple.

Ordinary aluminum electrolytic capacitors are inexpensive, but

often have poor Equivalent Series Resistance (ESR) and Equiva-

lent Series Inductance (ESL). Low ESR aluminum capacitors,

specifically designed for switch mode converter applications, are

also available, and these are a better choice than general purpose

devices. Even better performance can be achieved with tantalum

capacitors, although their cost is higher. Very low values of ESR

can be achieved by using OS-CON* capacitors (Sanyo Corpora-

tion, San Diego, CA). These devices are fairly small, available

with tape-and-reel packaging, and have very low ESR.

The effects of capacitor selection on output ripple are demon-

strated in Figures 12, 13, and 14. These figures show the output

of the same ADP1108 converter, which was evaluated with

three different output capacitors. In each case, the peak switch

current is 500 mA and the capacitor value is 100

µF. Figure 12

shows a Panasonic HF-series* radial aluminum electrolytic.

When the switch turns off, the output voltage jumps by about

90 mV and then decays as the inductor discharges into the ca-

pacitor. The rise in voltage indicates an ESR of about 0.18

. In

Figure 13, the aluminum electrolytic has been replaced by a

Sprague 593D-series* tantalum device. In this case the output

jumps about 35 mV, which indicates an ESR of 0.07

. Figure

14 shows an OS-CON SA series capacitor in the same circuit,

and ESR is only 0.02

.

*All trademarks are the property of their respective holders.