LM2679SX-ADJ National Semiconductor, LM2679SX-ADJ Datasheet - Page 14

• Manufacturer Part Number: LM2679SX-ADJ
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,BIPOLAR/MOS,SIP,7PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM2679SX-ADJ

• Rohs Compliant: NO

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Application Hints
where V
switch which is R
be typically 0.12Ω x 3.5A or 0.42V and V
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.
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
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
SAT
is the voltage drop across the internal power
ds(ON)
load
times I
max) indicates that L48 , a 47µH
load
(Continued)
. In this example this would
D
is the voltage drop
14
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.
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 Table 5 a 5A 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
current limit of at least 3.5A plus 50% or 5.25A.
Use a value of 7.15KΩ.
LLP PACKAGE DEVICES
The LM2679 is 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.
The Die Attach Pad (DAP) can and should be connected to
PCB Ground plane/island. For CAD and assembly guide-
lines
power.national.com.
refer
to Application
ADJ
Note AN-1187
to provide a peak switch
at
http://