lm9072 National Semiconductor Corporation, lm9072 Datasheet - Page 11

• Manufacturer Part Number: lm9072
• Description: Dual Tracking Low-dropout System Regulator
• Manufacturer: National Semiconductor Corporation

Application Information
Input Stability
Low dropout voltage regulators which utilize a PNP power
transistor usually exhibit a large increase in current when in
dropout (V
ration characteristics ( reduction) of the PNP transistor. To
significantly minimize this increase in current the LM9072
detects when the PNP enters saturation and reduces the
operating current.
This reduction in input current can create a stability problem
in applications with higher load current (
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 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
IN
<
5.5V). This increase is caused by the satu-
(Continued)
>
200 mA) where
FIGURE 6. Safety Latch-OFF
FIGURE 7. Input Bypassing
11
time of the input supply is very slow. If the application and
removal 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.
IN
of 3V to 5.5V) too quickly for an oscillation to
DS012906-23
DS012906-22
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