APPLICATION INFORMATION
The LM117/217/317 provides an internal
reference voltage of 1.25V between the output
and adjustments terminals. This is used to set a
constant current flow across an external resistor
divider (see fig. 4), giving an output voltage V
O
of:
V
O
=V
REF
(1 +
R
2
R
1
) +I
ADJ
R
2
The device was designed to minimize the term
I
ADJ
(100µA max) and to maintain it very constant
with line and load changes. Usually, the error
term I
ADJ
⋅ R
2
can be neglected. To obtain the
previous requirement, all the regulator quiescent
current is returned to the output terminal,
imposing a minimum load current condition. If the
load is insufficient, the output voltage will rise.
Since the LM117/217317 is a floating regulator
and ”sees” only the input-to-output differential
voltage,supplies of very high voltage with respect
to ground can be regulated as long as the
maximum input-to-output differential is not
exceeded. Furthermore, programmable regulator
are easily obtainable and, by connecting a fixed
resistor between the adjustment and output, the
device can be used as a precision current
regulator.
In order to optimise the load regulation, the
current set resistor R1 (see fig. 4) should be tied
as close as possible to the regulator, while the
ground terminal of R2 should be near the ground
of the load to provideremote ground sensing.
Performance may be improved with added
capacitanceas follow:
An input bypass capacitorof 0.1µF
An adjustment terminal to ground 10µF capacitor
Figure 4 : Basic Adjustable Regulator.
Figure 1 :
Output Current vs. Input-output
Differential Voltage.
Figure 2 :
Dropout Voltage vs. Junction
Temperature.
Figure 3 : Reference Voltage vs. Junction
LM117/217/317
4/11
