LM317 datasheet(9/25 Pages) STMICROELECTRONICS | 1.2 V to 37 V adjustable voltage regulators

9 page

LM117/LM217/LM317

Application information

9/25

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 VO of:

VO = VREF (1 + R2/R1) + 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 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 Figure 3) 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 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.

Note:

D1 protect the device against input short circuit, while D2 protects against output short

circuit for capacitors discharging.

Figure 7.

Voltage regulator with protection diodes

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