LM140K

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SNVS994 – JULY 2013

APPLICATION HINTS

The LM140K 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 17) 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 17 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 18)

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 17. Input Short

Figure 18. Regulator Floating Ground

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