LM117/LM217/LM317
Application information
6
Application information
The LM117/217/317 provides an internal reference voltage of 1.25 V between the output
and adjustments terminals. This is used to set a constant current flow across an external
resistor divider (see
Figure 3),
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
(100 µA 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
Figure 3)
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 0.1 µF
An adjustment terminal to ground 10 µF capacitor to improve the ripple rejection of about 15
dB (CADJ).
An 1 µF tantalum (or 25 µF Aluminium 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
Figure 4
D1 protect the device against input short circuit, while D2
protect against output short circuit for capacitance discharging.
Figure 7.
Voltage regulator with protection diodes
Note:
D1 protect the device against input short circuit, while D2 protects against output short
circuit for capacitors discharging.
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