LM2679T-ADJ National Semiconductor, LM2679T-ADJ Datasheet - Page 13

• Manufacturer Part Number: LM2679T-ADJ
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2679T-ADJ

• Input Voltage: 40V
• Output Current: 5A
• Output Voltage: 37V
• No. Of Outputs: 1
• No. Of Pins: 7
• Termination Type: Through Hole
• Mounting Type: Through Hole
• Voltage Regulator Type: Switching
• Output Current Max: 5A
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Hints
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
A recommended value to use for R1 is 1K. In this example
then R2 is determined to be:
R2 = 11.23K
The closest standard 1% tolerance value to use is 11.3K
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
Volt • microsecond constant (E • T expressed in V • µS) from
the following formula:
where V
switch which is R
be typically 0.12 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 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
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.
6 ,
SAT
FB
requires
is the feedback voltage of typically 1.21V.
is the voltage drop across the internal power
ds(ON)
a
load
times I
max) indicates that L48 , a 47µH
calculation
load
(Continued)
. In this example this would
D
of
is the voltage drop
the
inductor
13
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 7.15K .
ADJ
to provide a peak switch
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