Input Stability

Low dropout voltage regulators which utilize a PNP power

transistor usually exhibit a large increase in current when in

dropout (V

tion characteristics (

β reduction) of the PNP transistor. To

significantly minimize this increase in current the LM9072

detects when the PNP enters saturation and reduces the op-

erating current.

This reduction in input current can create a stability problem

the input voltage is applied through a long length of wire

which in effect add a significant amount of inductance in se-

ries with the input. The drop in input current may create a

positive input voltage transient which may take the PNP out

of saturation. If the input voltage is held constant at the

threshold where the PNP is going in and out of saturation, an

oscillation may be created.

This is only observed where a large series inductance is

present in the input supply line and when the rise and fall

time of the input supply is very slow. If the application and re-

moval of the input voltage changes at a rate greater than

500 mV/ms it will move through the dropout region of the

regulator (V

be established.

In the event that an oscillation is present, input bypassing

can also help de-tune the resonance.

Figure 7 illustrates two

input bypassing approaches. The straightforward addition of

a larger valued electrolytic capacitor could suffice. in this

case however, if reverse battery connections are a possibility

it is necessary to add a series protection diode as shown to

prevent damaging the polarized input capacitor.

An alternative input bypassing scheme is also shown. This

eliminates the use of polarized input capacitors and a series

protection diode. The values shown were derived empirically

in a representative typical application. Appropriate values for

any given application require experimentation.

DS012906-22

FIGURE 6. Safety Latch-OFF

DS012906-23

FIGURE 7. Input Bypassing

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