LM2673SD-5.0/NOPB National Semiconductor, LM2673SD-5.0/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM2673SD-5.0/NOPB
• Description: IC REG SIMPLE SWITCHER 14-LLP
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM2673SD-5.0/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 3A
• Frequency - Switching: 260kHz
• Voltage - Input: 8 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-LLP
• Primary Input Voltage: 12V
• No. Of Outputs: 1
• Output Voltage: 37V
• Output Current: 3A
• No. Of Pins: 14
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Input Voltage Primary Max: 40V
• Rohs Compliant: Yes
• For Use With: LM2673-5.0EVAL - EVALUATION BOARD FOR LM2673-5.0
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM2673SD-5.0; LM2673SD-5.0TR

least 30% inductor ripple current (±15% of Iout). For 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
±2% at room temperature and ±3% over the full operating
temperature range.
Step 3: Use the nomograph for the 3.3V device ,Figure 3. The
intersection of the 16V horizontal line (V
vertical line (I
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 ca-
pacitors 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 fol-
lowing 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 applica-
tion 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
t
V
V
V
V
Using Vin max ensures that the softstart delay time will be at
least the desired 50mS.
SST
SS
SST
OUT
SCHOTTKY
IN
: 50mS
: 16V
: 3.7μA
: 0.63V
: 3.3V
: 0.5V
out
load
max).
max) indicates that L33, a 22μH inductor, is
in
max) and the 2.5A
load
max). Again
13
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 re-
quired 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, Fig-
ure 6, requires a calculation of the inductor 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.
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:
SAT
FB
is the feedback voltage of typically 1.21V.
is the voltage drop across the internal power
ds(ON)
times I
load
ADJ
. In this example this would
to provide a peak switch
D
is the voltage drop
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