LM2678 datasheet(14/27 Pages) NSC

14 / 27 page

Application Hints (Continued)

The switching frequency of 260KHz is the nominal value to

use to estimate the ON time of the switch during which

energy is stored in the inductor.

For this example E

•T is found to be:

Using Figure 6, the intersection of 27V

•µS horizontally and

the 3.5A vertical line (I

load max) indicates that L48 , a 47µH

inductor, or L49, a 33µH inductor could be used. Either

inductor will be suitable, but for this example selecting the

larger inductance will result in lower ripple current.

From Table 1, L48 in a surface mount component is available

from Pulse Engineering with part number P0848.

Step 4: Use Table 6 to determine an output capacitor. With a

14.8V output the 12.5 to 15V row is used and with a 47µH

inductor there are three surface mount output capacitor so-

lutions. Table 2 provides the actual capacitor characteristics

based on the C Code number. Any of the following choices

can be used:

1 x 33µF/20V AVX TPS (code C6)

1 x 47µF/20V Sprague 594 (code C8)

1 x 47µF/20V Kemet T495 (code C8)

Important Note: When using the adjustable device in low

voltage applications (less than 3V output), if the nomograph,

Figure 6, selects an inductance of 22µH or less, Table 6 does

not provide an output capacitor solution. With these condi-

tions the number of output capacitors required for stable

operation becomes impractical. It is recommended to use

either a 33µH or 47µH inductor and the output capacitors

from Table 6.

LM2678 Datasheet(PDF) 14 Page - National Semiconductor (TI)