LP2989 Datasheet, PDF (16/19 Pages) National Semiconductor (TI) – Micropower/Low Noise, 500 mA Ultra Low-Dropout Regulator For Use with Ceramic Output Capacitors

Application Hints (Continued)
at high ambient temperatures (which could cause the
LP2989 to oscillate). Another significant problem with Z5U
and Y5V dielectric devices is that the capacitance drops
severely with applied voltage. A typical Z5U or Y5V capacitor
can lose 60% of its rated capacitance with half of the rated
voltage applied to it.
For these reasons, X7R and X5R type ceramic capaci-
tors must be used on the input and output of the
LP2989.
TANTALUM: Tantalum capacitors are less desirable than
ceramics for use as output capacitors because they are
typically more expensive when comparing equivalent capaci-
tance and voltage ratings in the 1 µF to 4.7 µF range.
Another important consideration is that Tantalum capacitors
have higher ESR values than equivalent size ceramics. This
means that while it may be possible to find a Tantalum
capacitor with an ESR value within the stable range, it would
have to be larger in capacitance (which means bigger and
more costly) than a ceramic capacitor with the same ESR
value.
It should also be noted that the ESR of a typical Tantalum will
increase about 2:1 as the temperature goes from 25˚C down
to −40˚C, so some guard band must be allowed.
Tantalum capacitors may be used on the input as long as the
requirement for minimum capacitance is met.
FILM: Polycarbonate and polypropelene film capacitors
have excellent electrical performance: their ESR is the low-
est of the three types listed, their capacitance is very stable
with temperature, and DC leakage currrent is extremely low.
One disadvantage is that film capacitors are larger in physi-
cal size than ceramic or tantalum which makes film a poor
choice for either input or output capacitors.
However, their low leakage makes them a good choice for
the noise bypass capacitor. Since the required amount of
capacitance is only .01 µF, small surface-mount film capaci-
tors are avalable in this size.
SHUTDOWN INPUT OPERATION
The LP2989 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 in the
Electrical Characteristics section under V_ON/OFF.
To prevent mis-operation, the turn-on (and turn-off) voltage
signals applied to the Shutdown input must have a slew rate
which is ≥ 40 mV/µs.
CAUTION: the regulator output voltage can not be guaran-
teed if a slow-moving AC (or DC) signal is applied that is in
the range between the specified turn-on and turn-off volt-
ages listed under the electrical specification V_ON/OFF (see
Electrical Characteristics).
REVERSE INPUT-OUTPUT VOLTAGE
The PNP power transistor used as the pass element in the
LP2989 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
reverse voltage across the LP2989 to 0.3V (see Absolute
Maximum Ratings).
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