Datasheet

LM217L/LM317L
4/10
Figure 1 : Current Limit Figure 2 : Minimum Operating Current
APPLICATION INFORMATION
The LM317L 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 LM317L 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.
Figure 3 : Basic Adjustable Regulator