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LM340, LM340A, LM7805, LM7812, LM7815

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SNOSBT0K – FEBRUARY 2000 – REVISED JULY 2016

Product Folder Links: LM340 LM340A LM7805 LM7812 LM7815

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Copyright © 2000–2016, Texas Instruments Incorporated

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LM340x and LM78xx series is designed with thermal protection, output short-circuit protection, and output

transistor safe area protection. However, as with any IC regulator, it becomes necessary to take precautions to

assure that the regulator is not inadvertently damaged. The following describes possible misapplications and

methods to prevent damage to the regulator.

8.1.1 Shorting the Regulator Input

When using large capacitors at the output of these regulators, a protection diode connected input to output

(Figure 15) may be required if the input is shorted to ground. Without the protection diode, an input short causes

stored charge in the large output capacitor. The capacitor will then discharge through a large internal input to

output diode and parasitic transistors. If the energy released by the capacitor is large enough, this diode, low

current metal, and the regulator are destroyed. The fast diode in Figure 15 shunts most of the capacitors

discharge current around the regulator. Generally no protection diode is required for values of output capacitance

≤ 10 μF.

8.1.2 Raising the Output Voltage Above the Input Voltage

Because the output of the device does not sink current, forcing the output high can cause damage to internal low

current paths in a manner similar to that just described in Shorting the Regulator Input.

8.1.3 Regulator Floating Ground

When the ground pin alone becomes disconnected, the output approaches the unregulated input, causing

occur to the regulator. This fault is most likely to occur when plugging in regulators or modules with on card

regulators into powered up sockets. The power must be turned off first, the thermal limit ceases operating, or the

ground must be connected first if power must be left on. See Figure 16.

8.1.4 Transient Voltages

If transients exceed the maximum rated input voltage of the device, or reach more than 0.8 V below ground and

have sufficient energy, they will damage the regulator. The solution is to use a large input capacitor, a series

input breakdown diode, a choke, a transient suppressor or a combination of these.

Figure 15. Input Short