Equivalent Schematic

Application Hints

The LM340/LM78XX series is designed with thermal protec-

tion, output short-circuit protection and output transistor safe

area protection. However, as with

any IC regulator, it be-

comes necessary to take precautions to assure that the

regulator is not inadvertently damaged. The following de-

scribes possible misapplications and methods to prevent

damage to the regulator.

Shorting the Regulator Input: When using large capacitors

at the output of these regulators, a protection diode con-

nected input to output (

Figure 1) may be required if the input

is shorted to ground. Without the protection diode, an input

short will cause the input to rapidly approach ground poten-

tial, while the output remains near the initial V

the stored charge in the large output capacitor. The capacitor

will then discharge through a large internal input to output di-

ode and parasitic transistors. If the energy released by the

capacitor is large enough, this diode, low current metal and

the regulator will be destroyed. The fast diode in

Figure 1 will

shunt most of the capacitors discharge current around the

regulator. Generally no protection diode is required for val-

ues of output capacitance

≤ 10 µF.

Raising the Output Voltage above the Input Voltage:

Since the output of the device does not sink current, forcing

Line Regulation

140AK-5.0, I

DS007781-5

Line Regulation

140AK-5.0, V

DS007781-6

DS007781-7

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