LM340-N, LM78xx
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SNOSBT0I –FEBRUARY 2000–REVISED MARCH 2013
APPLICATION HINTS
The LM340-N/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.
SHORTING THE REGULATOR INPUT
When using large capacitors at the output of these regulators, a protection diode connected input to output
(Figure 21) 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 potential, while the output remains near the initial VOUTbecause of the
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 will be destroyed. The fast diode in Figure 21 will shunt most of the capacitors
discharge current around the regulator. Generally no protection diode is required for values of output capacitance
≤ 10 μF.
RAISING THE OUTPUT VOLTAGE ABOVE THE INPUT VOLTAGE
Since 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 the “Shorting the Regulator Input” section.
REGULATOR FLOATING GROUND (Figure 22)
When the ground pin alone becomes disconnected, the output approaches the unregulated input, causing
possible damage to other circuits connected to VOUT. If ground is reconnected with power “ON”, damage may
also 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. Power should be turned off first, thermal limit ceases operating, or ground
should be connected first if power must be left on.
TRANSIENT VOLTAGES
If transients exceed the maximum rated input voltage of the device, or reach more than 0.8V 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 21. Input Short
Figure 22. Regulator Floating Ground
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