LM9073T National Semiconductor, LM9073T Datasheet - Page 14

• Manufacturer Part Number: LM9073T
• Description: Dual High Current Low-Dropout System Regulator
• Manufacturer: National Semiconductor

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Application Information
Input Stability
Low dropout voltage regulators which utilize a PNP power
transistor usually exhibit a large increase in current when in
dropout (Vin
tion characteristics ( 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 (
input voltage is applied through a long length of wire which in
effect add a significant amount of inductance in series with
Figure 6. Simple Logic Configuration to Provide
<
5.5V). This increase is caused by the satura-
ON/OFF Sensing
>
200mA). where the
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DS101296-21
14
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.