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PDF 2576 Data sheet ( Hoja de datos )
| Número de pieza | 2576 | |
| Descripción | 5-AmpAdjustableRegulators | |
| Fabricantes | NationalSemiconductor | |
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May 1998
LM138/LM338
5-Amp Adjustable Regulators
General Description
The LM138 series of adjustable 3-terminal positive voltage
regulators is capable of supplying in excess of 5A over a
1.2V to 32V output range. They are exceptionally easy to
use and require only 2 resistors to set the output voltage.
Careful circuit design has resulted in outstanding load and
line regulation — comparable to many commercial power
supplies. The LM138 family is supplied in a standard 3-lead
transistor package.
A unique feature of the LM138 family is time-dependent cur-
rent limiting. The current limit circuitry allows peak currents
of up to 12A to be drawn from the regulator for short periods
of time. This allows the LM138 to be used with heavy tran-
sient loads and speeds start-up under full-load conditions.
Under sustained loading conditions, the current limit de-
creases to a safe value protecting the regulator. Also in-
cluded on the chip are thermal overload protection and safe
area protection for the power transistor. Overload protection
remains functional even if the adjustment pin is accidentally
disconnected.
Normally, no capacitors are needed unless the device is situ-
ated more than 6 inches from the input filter capacitors in
which case an input bypass is needed. An output capacitor
can be added to improve transient response, while bypass-
ing the adjustment pin will increase the regulator’s ripple re-
jection.
Besides replacing fixed regulators or discrete designs, the
LM138 is useful in a wide variety of other applications. Since
the regulator is “floating” and sees only the input-to-output
differential voltage, supplies of several hundred volts can be
regulated as long as the maximum input to output differential
is not exceeded, i.e., do not short-circuit output to ground.
The part numbers in the LM138 series which have a K suffix
are packaged in a standard Steel TO-3 package, while those
with a T suffix are packaged in a TO-220 plastic package.
The LM138 is rated for −55˚C ≤ TJ ≤ +150˚C, and the LM338
is rated for 0˚C ≤ TJ ≤ +125˚C.
Features
n Guaranteed 7A peak output current
n Guaranteed 5A output current
n Adjustable output down to 1.2V
n Guaranteed thermal regulation
n Current limit constant with temperature
n P+ Product Enhancement tested
n Output is short-circuit protected
Applications
n Adjustable power supplies
n Constant current regulators
n Battery chargers
Connection Diagrams (See Physical Dimension section for further information)
(TO-3 STEEL)
Metal Can Package
(TO-220)
Plastic Package
DS009060-30
Bottom View
Order Number LM138K STEEL or LM338K STEEL
See NS Package Number K02A
DS009060-31
Front View
Order Number LM338T
See NS Package Number T03B
© 1999 National Semiconductor Corporation DS009060
www.national.com
1 page  
Application Hints (Continued)
FIGURE 1.
DS009060-6
Since the 50 µA current from the adjustment terminal repre-
sents an error term, the LM138 was designed to minimize
IADJ and make it very constant with line and load changes.
To do this, all quiescent operating current is returned to the
output establishing a minimum load current requirement. If
there is insufficient load on the output, the output will rise.
External Capacitors
An input bypass capacitor is recommended. A 0.1 µF disc or
1 µF solid tantalum on the input is suitable input bypassing
for almost all applications. The device is more sensitive to
the absence of input bypassiing when adjustment or output
capacitors are used but the above values will eliminate the
possiblity of problems.
The adjustment terminal can be bypassed to ground on the
LM138 to improve ripple rejection. This bypass capacitor
prevents ripple from being amplified as the output voltage is
increased. With a 10 µF bypass capacitor 75 dB ripple rejec-
tion is obtainable at any output level. Increases over 20 µF
do not appreciably improve the ripple rejection at frequen-
cies above 120 Hz. If the bypass capacitor is used, it is
sometimes necessary to include protection diodes to prevent
the capacitor from discharging through internal low current
paths and damaging the device.
In general, the best type of capacitors to use are solid tanta-
lum. Solid tantalum capacitors have low impedance even at
high frequencies. Depending upon capacitor construction, it
takes about 25 µF in aluminum electrolytic to equal 1 µF
solid tantalum at high frequencies. Ceramic capacitors are
also good at high frequencies; but some types have a large
decrease in capacitance at frequencies around 0.5 MHz. For
this reason, 0.01 µF disc may seem to work better than a
0.1 µF disc as a bypass.
Although the LM138 is stable with no output capacitors, like
any feedback circuit, certain values of external capacitance
can cause excessive ringing. This occurs with values be-
tween 500 pF and 5000 pF. A 1 µF solid tantalum (or 25 µF
aluminum electrolytic) on the output swamps this effect and
insures stability.
Load Regulation
The LM138 is capable of providing extremely good load
regulation but a few precautions are needed to obtain maxi-
mum performance. The current set resistor connected be-
tween the adjustment terminal and the output terminal (usu-
ally 240Ω) should be tied directly to the output of the
regulator (case) rather than near the load. This eliminates
line drops from appearing effectively in series with the refer-
ence and degrading regulation. For example, a 15V regula-
tor with 0.05Ω resistance between the regulator and load will
have a load regulation due to line resistance of 0.05Ω x IL. If
the set resistor is connected near the load the effective line
resistance will be 0.05Ω (1 + R2/R1) or in this case, 11.5
times worse.
Figure 2 shows the effect of resistance between the regula-
tor and 240Ω set resistor.
DS009060-7
FIGURE 2. Regulator with Line
Resistance in Output Lead
With the TO-3 package, it is easy to minimize the resistance
from the case to the set resistor, by using 2 separate leads to
the case. The ground of R2 can be returned near the ground
of the load to provide remote ground sensing and improve
load regulation.
Protection Diodes
When external capacitors are used with any IC regulator it is
sometimes necessary to add protection diodes to prevent
the capacitors from discharging through low current points
into the regulator. Most 20 µF capacitors have low enough
internal series resistance to deliver 20A spikes when
shorted. Although the surge is short, there is enough energy
to damage parts of the IC.
When an output capacitor is connected to a regulator and
the input is shorted, the output capacitor will discharge into
the output of the regulator. The discharge current depends
on the value of the capacitor, the output voltage of the regu-
lator, and the rate of decrease of VIN. In the LM138 this dis-
charge path is through a large junction that is able to sustain
25A surge with no problem. This is not true of other types of
positive regulators. For output capacitors of 100 µF or less at
output of 15V or less, there is no need to use diodes.
The bypass capacitor on the adjustment terminal can dis-
charge through a low current junction. Discharge occurs
when either the input or output is shorted. Internal to the
LM138 is a 50Ω resistor which limits the peak discharge cur-
rent. No protection is needed for output voltages of 25V or
less and 10 µF capacitance. Figure 3 shows an LM138 with
protection diodes included for use with outputs greater than
25V and high values of output capacitance.
5 www.national.com
5 Page 
Typical Applications (Continued)
0 to 22V Regulator
* R1 = 240Ω, R2 = 5k for LM138
Full output current not available
at high input-output voltages
DS009060-19
12V Battery Charger
DS009060-20
11 www.national.com
11 Page | ||
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