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 regula-
tors, a protection diode connected 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 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 1 will shunt most of the capacitors
discharge current around the regulator. Generally no protec-
tion diode is required for values of output capacitance ≤ 10
µF.
00778108
FIGURE 1. Input Short
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 “Short-
ing the Regulator Input” section.
00778109
FIGURE 2. Regulator Floating Ground
REGULATOR FLOATING GROUND (Figure 2)
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 solu-
tion is to use a large input capacitor, a series input break-
down diode, a choke, a transient suppressor or a combina-
tion of these.
00778110
FIGURE 3. Transients
When a value for θ(H–A) is found using the equation shown,
a heatsink must be selected that has a value that is less than
or equal to this number.
θ(H–A) is specified numerically by the heatsink manufacturer
in this catalog, or shown in a curve that plots temperature
rise vs power dissipation for the heatsink.
11
www.national.com
NSC [ National Semiconductor ]
