LM317K datasheet(4/11 Pages) STMICROELECTRONICS

4 / 11 page

APPLICATION INFORMATION

The

LM117/217/317

provides

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 +

) +IADJ R2

The device was designed to minimize the term

IADJ (100

µA 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 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

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 optimise 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.

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

Figure 4 : Basic Adjustable Regulator.

Figure 1 : Output Current vs. Input-output

Differential Voltage.

Figure 2 : Dropout Voltage vs. Junction

Temperature.

Figure 3 : Reference Voltage vs. Junction

LM117/217/317

4/11

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