LM117, LM217, LM317

Application information

Doc ID 2154 Rev 15

9/25

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

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

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