LM217M_08 datasheet(9/21 Pages) STMICROELECTRONICS

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LM217M - LM317M

Application information

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Application information

The LM217M/LM317M provide an internal reference voltage of 1.25 V between the output

and adjustment terminals. These devices are used to set a constant current flow across an

external resistor divider (see Figure 6), giving an output voltage VO of: VO = VREF (1 + R2 /

R1) + IADJ R2.

The devices were designed to minimize the term IADJ (100 µA max) and to maintain it very

constant in 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 LM217M/LM317M devices are floating regulators and "see" 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 obtained and, by connecting a fixed resistor between

the adjustment and output, the devices can be used as a precision current regulator. In

order to optimize the load regulation, the current set resistor R1 (see Figure 6) 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.

6.1

External capacitors (

Figure 8)

Normally no capacitors are needed unless the devices are situated far from the input filter

capacitors; in which case an input bypass is needed.

A 0.1 µF disc or 1 µF tantalum input bypass capacitor (CI) is recommended to reduce the

sensitivity to input line impedance.

The adjustment terminal may be bypassed to ground to improve ripple rejection. This

capacitor (CADJ) prevents ripple from being amplified as the output voltage is increased. A

10 µF capacitor should improve ripple rejection of about 80 dB at 120 Hz in a 10 V

application.

Although the LM217M/LM317M devices are stable with no output capacitance like any

feedback circuit, certain values of external capacitance can cause excessive ringing. An

output capacitance (CO) in the form of a 1 µF tantalum or 25 µF aluminium electrolytic

capacitor on the output swamps this effect and insures stability.

6.2

Protection diodes (

Figure 8)

When external capacitors are used with any IC regulator it is sometimes necessary to add

protection diodes to prevent the capacitors from discharging through low current points into

the regulator.

Figure 6 shows the LM217M/LM317M 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

It’s not guaranteed the Re-Boot of the device when the junction temperature is over 85 °C.

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