Datasheet

LM117/217/317
5/15
Figure 6: Reference Voltage vs Junction
Figure 7: Basic Adjustable Regulator
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 optimize
the load regulation, the current set resistor R
1
(see fig. 4) should be tied as close as possible to the
regulator, while the ground terminal of R
2
should be near the ground of the load to provide remote ground
sensing. Performance may be improved with added capacitance as follow:
An input bypass capacitor of 0.1µF
An adjustment terminal to ground 10µF capacitor to improve the ripple rejection of about 15 dB (C
ADJ
).
An 1µF tantalum (or 25µFAluminium electrolytic) capacitor on the output to improve transient response. In
additional to external capacitors, it is good practice to add protection diodes, as shown in fig.5. D1 protect
the device against input short circuit, while D2 protect against output short circuit for capacitance
discharging.