LM2673S-12 National Semiconductor, LM2673S-12 Datasheet - Page 13

• Manufacturer Part Number: LM2673S-12
• Description: IC, SWITCHING REG 3A 12V, SMD, 2673
• Manufacturer: National Semiconductor

Specifications of LM2673S-12

• Primary Input Voltage: 24V
• No. Of Outputs: 1
• Output Voltage: 12V
• Output Current: 3A
• Voltage Regulator Case Style: TO-263
• No. Of Pins: 7
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Application Hints
Step 7: Include a 0.01µF/50V capacitor for Cboost in the
design and then determine the value of a softstart capacitor
if desired.
Step 8: Define a value for R
current limit to be at least 20% greater than Iout max to allow
for at least 30% inductor ripple current (
designs that must operate over the full temperature range
the switch current limit should be set to at least 50% greater
than Iout max (1.5 x I
FIXED OUTPUT VOLTAGE DESIGN EXAMPLE
A system logic power supply bus of 3.3V is to be generated
from a wall adapter which provides an unregulated DC volt-
age of 13V to 16V. The maximum load current is 2.5A. A
softstart delay time of 50mS is desired. Through-hole com-
ponents are preferred.
Step 1: Operating conditions are:
Vout = 3.3V
Vin max = 16V
Iload max = 2.5A
Step 2: Select an LM2673T-3.3. The output voltage will have
a tolerance of
temperature range.
Step 3: Use the nomograph for the 3.3V device ,Figure 3.
The intersection of the 16V horizontal line (V
2.5A vertical line (I
inductor, is required.
From Table 1, L33 in a through-hole component is available
from Renco with part number RL-1283-22-43 or part number
PE-53933 from Pulse Engineering.
Step 4: Use Table 3 to determine an output capacitor. With a
3.3V output and a 33µH inductor there are four through-hole
output capacitor solutions with the number of same type
capacitors to be paralleled and an identifying capacitor code
given. Table 2 provides the actual capacitor characteristics.
Any of the following choices will work in the circuit:
1 x 220µF/10V Sanyo OS-CON (code C5)
1 x 1000µF/35V Sanyo MV-GX (code C10)
1 x 2200µF/10V Nichicon PL (code C5)
1 x 1000µF/35V Panasonic HFQ (code C7)
Step 5: Use Table 4 to select an input capacitor. With 3.3V
output and 22µH there are three through-hole solutions.
These capacitors provide a sufficient voltage rating and an
rms current rating greater than 1.25A (1/2 I
using Table 2 for specific component characteristics the
following choices are suitable:
1 x 1000µF/63V Sanyo MV-GX (code C14)
1 x 820µF/63V Nichicon PL (code C24)
1 x 560µF/50V Panasonic HFQ (code C13)
Step 6: From Table 5 a 3A or more Schottky diode must be
selected. The 20V rated diodes are sufficient for the appli-
cation and for through-hole components two part types are
suitable:
1N5820
SR302
Step 7: A 0.01µF capacitor will be used for Cboost. For the
50mS softstart delay the following parameters are to be
used:
I
±
SST
2% at room temperature and
: 3.7µA
load
out
max) indicates that L33, a 22µH
max).
ADJ
(Continued)
±
3% over the full operating
to set the peak switch
±
15% of Iout). For
load
in
max) and the
max). Again
13
t
V
V
V
V
Using Vin max ensures that the softstart delay time will be at
least the desired 50mS.
Using the formula for Css a value of 0.148µF is determined
to be required. Use of a standard value 0.22µF capacitor will
produce more than sufficient softstart delay.
Step 8: Determine a value for R
current limit of at least 2.5A plus 50% or 3.75A.
Use a value of 10KΩ.
ADJUSTABLE OUTPUT DESIGN EXAMPLE
In this example it is desired to convert the voltage from a two
battery automotive power supply (voltage range of 20V to
28V, typical in large truck applications) to the 14.8VDC alter-
nator supply typically used to power electronic equipment
from single battery 12V vehicle systems. The load current
required is 2A maximum. It is also desired to implement the
power supply with all surface mount components. Softstart is
not required.
Step 1: Operating conditions are:
Vout = 14.8V
Vin max = 28V
Iload max = 2A
Step 2: Select an LM2673S-ADJ. 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:
Where V
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 3: To use the nomograph for the adjustable device,
Figure
Volt • microsecond constant (E • T expressed in V • µS) from
the following formula:
where V
switch which is R
be typically 0.15Ω x 2A or 0.3V and V
across the forward bisased Schottky diode, typically 0.5V.
SS
SST
OUT
SCHOTTKY
IN
: 50mS
: 16V
: 0.63V
: 3.3V
SAT
FB
6,
: 0.5V
is the feedback voltage of typically 1.21V.
is the voltage drop across the internal power
requires
ds(ON)
times I
a
calculation
load
ADJ
. In this example this would
to provide a peak switch
D
is the voltage drop
of
the
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inductor