LM2679 datasheet(13/24 Pages) NSC

13 / 24 page

Application Hints (Continued)

Step 2: Select an LM2679S-ADJ. To set the output voltage

to 14.9V two resistors need to be chosen (R1 and R2 in

Fig-

ure 2). For the adjustable device the output voltage is set by

the following relationship:

Where V

FB is the feedback voltage of typically 1.21V.

A recommended value to use for R1 is 1K. In this example

then R2 is determined to be:

R2 = 11.23K

Ω

The closest standard 1% tolerance value to use is 11.3K

Ω

This will set the nominal output voltage to 14.88V which is

within 0.5% of the target value.

Step 3: To use the nomograph for the adjustable device,

Fig-

ure

requires

calculation

the

inductor

Volt

•microsecond constant (E•T expressed in V•µS) from

the following formula:

where V

SAT is the voltage drop across the internal power

switch which is R

ds(ON) times Iload. In this example this would

be typically 0.12

Ω x 3.5A or 0.42V and V

D is the voltage drop

across the forward bisased Schottky diode, typically 0.5V.

The switching frequency of 260KHz is the nominal value to

use to estimate the ON time of the switch during which en-

ergy 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 in-

ductor 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 in-

ductor there are three surface mount output capacitor solu-

tions. 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 op-

eration becomes impractical. It is recommended to use ei-

ther a 33µH or 47µH inductor and the output capacitors from

Table 6.

Step 5: An input capacitor for this example will require at

least a 35V WV rating with an rms current rating of 1.75A

(1/2 Iout max). From Table 2 it can be seen that C12, a

33µF/35V

capacitor

from

Sprague,

has

the

highest

voltage/current rating of the surface mount components and

that two of these capacitor in parallel will be adquate.

Step 6: From Table5a5A or more Schottky diode must be

selected. For surface mount diodes with a margin of safety

on the voltage rating one of two diodes can be used:

MBRD1545CT

6TQ045S

Step 7: A 0.01µF capacitor will be used for Cboost.

The softstart pin will be left open circuited.

Step 8: Determine a value for R

ADJ to provide a peak switch

current limit of at least 3.5A plus 50% or 5.25A.

Use a value of 7.15K

Ω.

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