SNVS774L – MAY 2004 – REVISED FEBRUARY 2011
Typical Performance Characteristics (continued)
Output Capacitor = 0
μF
unless otherwise noted
Line Transient Response
Load Transient Response
Figure 24.
Figure 25.
APPLICATION HINTS
In operation, the LM117 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 100μA current from the adjustment terminal represents an error term, the LM117 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 0.1μF disc or 1μF solid tantalum on the input is suitable input
bypassing for almost all applications. The device is more sensitive to the absence of input bypassing when
adjustment or output capacitors are used but the above values will eliminate the possibility of problems.
The adjustment terminal can be bypassed to ground on the LM117 to improve ripple rejection. This bypass
capacitor prevents ripple from being amplified as the output voltage is increased. With a 10
μF
bypass capacitor
80dB ripple rejection is obtainable at any output level. Increases over 10
μF
do not appreciably improve the
ripple rejection at frequencies above 120Hz. 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.
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