LP2986 Datasheet, PDF (12/14 Pages) National Semiconductor (TI) – Micropower, 200 mA Ultra Low-Dropout Fixed or Adjustable Voltage Regulator

Application Hints (Continued)
A lead compensation capacitor (C_F) must also be used to
place a zero in the loop response at about 50 kHz. The value
for C _F can be found using:
C_F = 1/(2π x R1 x 50k)
A good quality capacitor must be used for C_F to ensure that
the value is accurate and does not change significantly over
temperature. Mica or ceramic capacitors can be used, as-
suming a tolerance of ±20% or better is selected.
If a ceramic is used, select one with a temperature coeffi-
cient of NPO, COG, Y5P, or X7R. Capacitor types Z5U, Y5V,
and Z4V can not be used because their value varies more
that 50% over the −25˚C to +85˚C temperature range.
SHUTDOWN INPUT OPERATION
The LP2986 is shut off by driving the Shutdown input low,
and turned on by pulling it high. If this feature is not to be
used, the Shutdown input should be tied to V_IN to keep the
regulator output on at all times.
To assure proper operation, the signal source used to drive
the Shutdown input must be able to swing above and below
the specified turn-on/turn-off voltage thresholds listed as V_H
and V_L, respectively (see Electrical Characteristics).
It is also important that the turn-on (and turn-off) voltage sig-
nals applied to the Shutdown input have a slew rate which is
not less than 40 mV/µs.
CAUTION: the regulator output state can not be guaranteed
if a slow-moving AC (or DC) signal is applied that is in the
range between V_H and V_L.
REVERSE INPUT-OUTPUT VOLTAGE
The PNP power transistor used as the pass element in the
LP2986 has an inherent diode connected between the regu-
lator output and input.
During normal operation (where the input voltage is higher
than the output) this diode is reverse-biased.
However, if the output is pulled above the input, this diode
will turn ON and current will flow into the regulator output.
In such cases, a parasitic SCR can latch which will allow a
high current to flow into V_IN (and out the ground pin), which
can damage the part.
In any application where the output may be pulled above the
input, an external Schottky diode must be connected from
V_IN to V_OUT (cathode on V_IN, anode on V_OUT), to limit the re-
verse voltage across the LP2986 to 0.3V (see Absolute
Maximum Ratings).
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