LM9073 datasheet(14/18 Pages) NSC

14 / 18 page

Application Information (Continued)

Input Stability

Low dropout voltage regulators which utilize a PNP power

transistor usually exhibit a large increase in current when in

dropout (Vin < 5.5V). This increase is caused by the satura-

tion characteristics (

β reduction) of the PNP transistor. To

significantly minimize this increase in current the LM9073

detects when the PNP enters the saturation and reduces the

operating current.

This reduction in input current can create a stability problem

in applications with higher load current (>200mA). where the

input voltage is applied through a long length of wire which in

effect add a significant amount of inductance in series with

the input. The drop in input current may create a positive in-

put voltage transient which may take the PNP out of satura-

tion. If the input voltage is held constant at the threshold

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

tion 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

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

regulator (Vin of 3V to 5.5V) too quickly for an oscillation to

be established.

Figure 6. Simple Logic Configuration to Provide

ON/OFF Sensing

DS101296-21

LM9073 Datasheet(PDF) 14 Page - National Semiconductor (TI)