LM2673S-5.0 National Semiconductor, LM2673S-5.0 Datasheet - Page 13

• Manufacturer Part Number: LM2673S-5.0
• Description: DC/DC Converter IC
• Manufacturer: National Semiconductor

Specifications of LM2673S-5.0

• Input Voltage: 40V
• Output Current: 3A
• Output Voltage: 5V
• No. Of Pins: 7
• Termination Type: SMD
• Mounting Type: Through Hole
• Voltage Regulator Type: Switching
• Output Current Max: 3A
• Peak Reflow Compatible (260 C): No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Hints
Step 2: Select an LM2673S-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 0.15
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 2A vertical line (I
ductor, should be used.
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 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 1A (1/2
6 ,
SAT
FB
requires
is the feedback voltage of typically 1.21V.
is the voltage drop across the internal power
ds(ON)
x 2A or 0.3V and V
load
a
times I
max) indicates that L38 , a 68µH in-
calculation
load
(Continued)
. In this example this would
D
of
is the voltage drop
the
inductor
13
Iout max). From Table 2 it can be seen that C12, a 33µF/35V
capacitor from Sprague, has the required voltage/current rat-
ing of the surface mount components.
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
30WQ04F
MBRS340
MBRD340
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 2A plus 50% or 3A.
Use a value of 12.4K .
ADJ
to provide a peak switch
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