6

Application information

The LM217M and LM317M provide an internal reference voltage (1.25 V) between the output and adjustment

terminals. These devices set a constant current flow across an external resistor divider (see Figure 6. Basic

adjustable regulator), giving the following output voltage:

Equation 1

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

These devices minimize the term IADJ (100 µA max.) and keep it constant with line and load changes. Usually,

the error terms: IADJ × R2 can be neglected. To obtain the previous requirement, the regulator quiescent current

is returned to the output terminal, imposing a minimum load current condition. If the load is insufficient, the output

voltage rises.

Since the LM217M and LM317M devices are floating regulators and only “see” the input-to-output differential

voltage, high voltage supplies can be regulated as long as the maximum input-to-output differential is not

exceeded. Furthermore, programmable regulators are easily obtained and, by connecting a fixed resistor between

the adjustment and output, the devices can be used as precision current regulators. In order to optimize the load

regulation, R1, the current set resistor (see Figure 6. Basic adjustable regulator) should be as closer as possible

to the regulator, while R2, the ground terminal should be near the ground of the load to provide remote ground

sensing.

6.1

External capacitors

Usually, capacitors are not necessary unless the devices are far from the input filter capacitors; in this case an

input bypass is needed.

To reduce the sensitivity to input line impedance, a 0.1 µF disc or 1 µF tantalum input bypass capacitor (CI) is

recommended.

The adjustment terminal may be bypassed to ground to improve ripple rejection. This capacitor (CADJ) avoids the

amplification of ripple as the output voltage rises. A 10 µF capacitor should improve ripple rejection about 80 dB at

120 Hz in a 10 V application.

Although the devices are stable without any output capacitors, some external capacitance values can cause

excessive ringing. A 1 µF solid tantalum or 25 µF aluminum electrolytic output capacitor swamps this effect and

assures stability.

6.2

Protection diodes

When external capacitors are used with any IC regulator, sometimes some protection diodes have to be added to

prevent the capacitors from discharging through low current points into the regulator.

Figure 8. Voltage regulator with protection diodes shows the devices with the recommended protection diodes for

output voltages in excess of 25 V or high capacitance values (C3 > 25 µF, C2 > 10 µF). Diode D1 prevents C3

from discharging through the IC during an input short-circuit. The combination of diodes D1 and D2 prevents C2

from discharging through the regulator during an input or output short-circuit.

6.3

Start-up block

Reboot of the device is not guaranteed when the junction temperature is over 85 °C.

LM217M, LM317M

Application information

DS0522 - Rev 12

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