LM217MDT-TR View Datasheet(PDF) - STMicroelectronics

Part Name

Description

View to exact match

LM217MDT-TR

Medium current 1.2 to 37 V adjustable voltage regulator

STMicroelectronics 

Application information

LM217M, LM317M

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.

10/19

DocID2577 Rev 9

Share Link: