Vin max = 28V
Iload max = 2A
From Table 1, L38 in a surface mount component is available
from Pulse Engineering with part number PE-54038S.
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 68μH
inductor 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:
Step 2: Select an LM2676S-ADJEP. To set the output voltage
to 14.9V two resistors need to be chosen (R1 and R2 in Figure
2). For the adjustable device the output voltage is set by the
following relationship:
1 x 33μF/20V AVX TPS (code C6)
1 x 47μF/20V Sprague 594 (code C8)
1 x 47μF/20V Kemet T495 (code C8)
Where VFB is the feedback voltage of typically 1.21V.
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 ei-
ther a 33μH or 47μH inductor and the output capacitors from
Table 6.
A recommended value to use for R1 is 1K. In this example
then R2 is determined to be:
R2 = 11.23KΩ
The closest standard 1% tolerance value to use is 11.3KΩ
This will set the nominal output voltage to 14.88V which is
within 0.5% of the target value.
Step 5: An input capacitor for this example will require at least
a 35V WV rating with an rms current rating of 1A (1/2 Iout
max). From Table 2 it can be seen that C12, a 33μF/35V ca-
pacitor from Sprague, has the required voltage/current rating
of the surface mount components.
Step 3: To use the nomograph for the adjustable device, Fig-
ure 6, requires a calculation of the inductor Volt•microsecond
constant (E•T expressed in V•μS) from the following formula:
Step 6: From Table 5 a 3A Schottky diode must be selected.
For surface mount diodes with a margin of safety on the volt-
age rating one of five diodes can be used:
SK34
30BQ040
where VSAT is the voltage drop across the internal power
switch which is Rds(ON) times Iload. In this example this would
be typically 0.15Ω x 2A or 0.3V and VD 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 energy
is stored in the inductor.
30WQ04F
MBRS340
MBRD340
Step 7: A 0.01μF capacitor will be used for Cboost.
LLP PACKAGE DEVICES
The LM2676EP may be offered in the 14 lead LLP surface
mount package to allow for a significantly decreased footprint
with equivalent power dissipation compared to the TO-263.
For details on mounting and soldering specifications, refer to
Application Note AN-1187 at http://power.national.com.
For this example E•T is found to be:
Using Figure 6, the intersection of 27V•μS horizontally and
the 2A vertical line (Iload max) indicates that L38 , a 68μH in-
ductor, should be used.
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200996 Version 2 Revision 3 Print Date/Time: 2011/03/24 16:39:24
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