LM117, LM217, LM317
Application information
6
Application information
The LM117, LM217, LM317 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
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, LM217, LM317 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
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
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.
Doc ID 2154 Rev 15
9/25
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