LM138/LM338
Application Hints
(Continued)
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 I
L
. 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.
shows the effect of resistance between the regula-
tor and 240Ω set resistor.
00906006
FIGURE 1.
Since the 50 µA current from the adjustment terminal repre-
sents an error term, the LM138 was designed to minimize
I
ADJ
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
rejection is obtainable at any output level. Increases over 20
µF do not appreciably improve the ripple rejection at fre-
quencies 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
tantalum. Solid tantalum capacitors have low impedance
even at high frequencies. Depending upon capacitor con-
struction, 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-
00906007
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
regulator, and the rate of decrease of V
IN
. In the LM138 this
discharge 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
current. No protection is needed for output voltages of 25V
or less and 10 µF capacitance.
shows an LM138
with protection diodes included for use with outputs greater
than 25V and high values of output capacitance.
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