LM337
The LM317 is a popular adjustable linear voltage regulator. It was designed by Robert C Dobkin in 1976 while he worked at National Semiconductor.
The LM337 (a negative complement to the LM317) regulates voltages below, rather than above, the reference. It was designed by Robert "Bob" Pease (1940 – 2011).
Without a heat sink with an ambient temperature at 50 ⁰C such as on a hot summer day inside a box, a maximum power dissipation of (TJ-TA)/RθJA = ((125-50)/80) = 0.98 W can be permitted. (A piece of shiny sheet metal of Aluminum with the dimensions 6 x 6 cm and 1.5 mm thick, results in a thermal resistance that permits 4.7 W of heat dissipation).
In a constant voltage mode with an input voltage source at VIN at 34 V and a desired output voltage of 5 V, the maximum output current will be PMAX / (VI-VO) = 0.98 / (34-5) = 32 mA.
For a constant current mode with an input voltage source at VIN at 12 V and a forward voltage drop of VF=3.6 V, the maximum output current will be PMAX / (VI - VF) = 0.98 / (12-3.6) = 112 mA.
As linear regulators, the LM317 and LM337 are used in DC to DC converter applications.
Linear regulators inherently waste power, the power dissipated is the current passed multiplied by the voltage difference between input and output. In use an LM317 commonly requires a heat sink to prevent the operating temperature rising too high. For large voltage differences, the energy lost as heat can ultimately be greater than that provided to the circuit. This is the trade-off for using linear regulators which are a simple way to provide a stable voltage with few additional components. The alternative is to use a switching voltage regulator which is usually more efficient but has a larger footprint and requires a larger number of associated components.
In packages with a heat-dissipating mounting tab, such as TO-220, the tab is connected internally to the output pin which may make it necessary to electrically isolate the tab or the heat sink from other parts of the application circuit. Failure to do this may cause the circuit to short.
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