What is the maximum input voltage of lm317

What is the maximum input voltage of lm317

Description:The LM317 is an adjustable linear voltage regulator capable of outputting voltages from 1.25 to 37V at a maximum of 1.5A with an input voltage range of 3 to 40V.

Package includes.

LM317 adjustable voltage regulator

Main features of lm317 adjustable voltage regulator.

Linear adjustable voltage regulator

Input voltage range 3 to 40V

Output voltage 1.25 - 37V

1.5 A constant current and 2.2 A peak current

TO-220 package

The LM317 is one of the most popular and oldest linear regulators. The input voltage can be varied from 3 to 40V and the output can be adjusted from 1.25V to 37V via the LM317 pin assignment; the output current is up to 1.5A. They have built-in current limiting and over-temperature protection and are generally quite robust devices.

The LM317 can be used in prototype designs to replace several fixed voltage regulators. They can also be easily wired to create a simple and inexpensive variable power supply for prototyping. We offer a small, simple module based on the LM317 that can be used for the same purpose.

Basic operation

The LM317 is a floating 3-pole regulator that is not grounded like most regulators. This allows it to regulate potentially very high voltages as long as the maximum input/output voltage of 40V is not exceeded.

Instead of a ground pin, it has a trimming pin that regulates the output voltage by means of a resistive voltage divider network between output and ground. These can be two fixed resistors if a constant output voltage is required, or one of the resistors can be a variable potentiometer so that the output can be regulated over a range.

Unlike a typical 78XX regulator, the LM317 requires a minimum load current to achieve full regulation. This is typically less than 10 milliamps, so this is not a problem in most applications. If this is a problem, a small load resistor can be added to the output to ensure a 10 milliamp draw.

The basic circuit does not need to have a bypass capacitor, but a 0.1uF ceramic input filter capacitor should be added if used at a distance from the input voltage source. If necessary, a 1.0uF or 20uF or larger tantalum output capacitor can also be added to improve transient response.

A basic circuit for connecting an LM317 with an adjustable output is shown below.

Example circuit diagram of the LM317

Power dissipation

Compared to DC-DC converters, linear regulators have lower output ripple and can be used for the same purpose, but the disadvantage is that linear regulators also tend to dissipate more heat. The reason is that linear regulators use a series-connected pass crystal at their output to remove excess voltage.

The power dissipation of a linear regulator depends on the difference between the input voltage (Vin) and output voltage (Vout) and the amount of current drawn by the regulator. The greater the voltage difference between Vin and Vout, the greater the power dissipation, which limits the amount of current that can be drawn from the device.

The power dissipation of the LM317 can be easily calculated as: Power dissipation = (Vin - Vout) * Iout.

If the LM317 has an input of 15 V and the output is set to 10 V and provides 1 A of current, the power dissipation = (15 V - 10 V) * 1 A = 5 W. In a TO-220 package, the LM317 chip should dissipate 5 W. Under normal conditions, the device can dissipate between 1 and 1.25 W before a heat sink is needed. Thus, in our example, the device would certainly need a heat sink. In this case, the maximum output current without a heatsink would be limited to about 250 to 300 mA and the device would operate in the 85 to 95°C temperature range.

On the other hand, if you run the LM317 with a 12V input, the power dissipated = (12V - 10) * 1A = 2 W. It's still hot, but it's much better than 5 W. Without a heat sink it could draw between 500 and 700 mA.

As a rule of thumb, always use the lowest possible input voltage to minimize power loss through the device and maximize the available output current.

Caution.

The LM317 label is connected to the output pins.

With high current loads or large voltage differences between input and output, the device can become very hot, so care must be taken when handling it.

TECHNICAL SPECIFICATIONS

Maximum Ratings
VIN	Max Input – Output V Differential	40V
IO	Maximum Output Current	1.5A (Typical)
IMAX	Peak Surge Current (typ)	2.2A
Operating Ratings
VO	Output Voltage	1.25V – 37V
VI – VO	Drop-out Voltage	3.0V (Max)  1.75V (Typ)
Package		TO-220
	Package Type	Plastic Tab, 3-lead, through hole
Manufacturer		ON Semiconductor
Datasheet		LM317