SNVS771B – MAY 1998 – REVISED APRIL 2013
APPLICATION HINTS
In operation, the LM138 develops a nominal 1.25V reference voltage, V
REF
, between the output and adjustment
terminal. The reference voltage is impressed across program resistor R1 and, since the voltage is constant, a
constant current I
1
then flows through the output set resistor R2, giving an output voltage of
(1)
Since the 50
μA
current from the adjustment terminal represents 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 frequencies 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 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 between 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
maximum performance. The current set resistor connected between the adjustment terminal and the output
terminal (usually 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 reference and degrading regulation. For
example, a 15V regulator with 0.05Ω resistance between the regulator and load will have a load regulation due to
line resistance of 0.05Ω × 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 regulator and 240Ω set resistor.
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