參數(shù)資料
型號(hào)： LM2673S-3.3
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 穩(wěn)壓器
英文描述： SIMPLE SWITCHER 3A Step-Down Voltage Regulator with Adjustable Current Limit
中文描述： 5.4 A SWITCHING REGULATOR, 280 kHz SWITCHING FREQ-MAX, PSSO7
封裝： TO-263, 7 PIN
文件頁(yè)數(shù)： 13/26頁(yè)
文件大?。?/td> 451K
代理商： LM2673S-3.3
Application Hints
(Continued)
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
ADJ
to set the peak switch
current limit to be at least 20% greater than Iout max to allow
for at 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
out
max).
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
in
max) and the
2.5A vertical line (I
load
max) indicates that L33, a 22μH
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
max). Again
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
: 3.7μA
t
SS
: 50mS
V
SST
: 0.63V
V
OUT
: 3.3V
V
SCHOTTKY
: 0.5V
V
IN
: 16V
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
ADJ
to provide a peak switch
current limit of at least 2.5A plus 50% or 3.75A.
Use a value of 10K
.
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
FB
is the feedback voltage of typically 1.21V.
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,
Figure
6
,
requires
a
calculation
Volt
microsecond constant (E
T expressed in V
μS) from
the following formula:
of
the
inductor
where V
is the voltage drop across the internal power
switch which is R
ds(ON)
times I
load
. In this example this would
be typically 0.15
is the voltage drop
across the forward bisased Schottky diode, typically 0.5V.
L
www.national.com
13
相關(guān)PDF資料
PDF描述
LM2673S-5.0 SIMPLE SWITCHER 3A Step-Down Voltage Regulator with Adjustable Current Limit
LM2673S-ADJ SIMPLE SWITCHER 3A Step-Down Voltage Regulator with Adjustable Current Limit
LM2673SD-12 SIMPLE SWITCHER 3A Step-Down Voltage Regulator with Adjustable Current Limit
LM2673SD-3.3 Miniature Switch, High Precision
LM2673SD-ADJ Miniature Switch, High Precision
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