LM317L datasheet(4/10 Pages) STMICROELECTRONICS

4 / 10 page

LM217L/LM317L

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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 VO of:

VO =VREF (1 + R2/R1)+ IADJ R2

The device was designed to minimize the term IADJ (100µA max) and to maintain it very constant with line

and load changes. Usually, the error term IADJ ×R2 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 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 provide remote ground sensing.

Figure 3 : Basic Adjustable Regulator

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