6

Application information

The 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 external resistor divider (see Figure 6. Basic

adjustable regulator), 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 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 regulators 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 6. Basic

adjustable regulator) 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 addition to external capacitors, it is good practice to add protection diodes, as shown in figure below D1

protect the device against input short circuit, while D2 protect against output short circuit for capacitance

discharging.

Figure 7. Voltage regulator with protection diodes

Input

Output

Adjust

Note:

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

discharging.

LM217, LM317

Application information

DS0433 - Rev 22

page 10/34