LP2980LV Datasheet, PDF (11/14 Pages) National Semiconductor (TI) – Micropower SOT, 50 mA Low-Voltage Low-Dropout Regulator For Applications With Output Voltages < 2V

Application Hints (Continued)
Another factor to consider is that Tantalum manufacturers
are presently designing their products toward the goal of
getting the lowest possible ESR, in an attempt to compete
with the new high-value ceramic capacitors. This means that
the typical values will probably continue to decline in the
future.
SELECTING THE OUTPUT CAPACITOR
This section contains guidelines for selecting an output ca-
pacitor which will maintain good regulator stability over the
entire operating temperature range (refer to ESR CURVES).
2.2 µF OUTPUT CAPACITOR
The smallest output capacitor which can be used with the
LP2980-1.8 is 2.2 µF. However, care must be exercised if
this value is used because of the ESR requirement.
At load currents ≤ 25mA, the stable ESR range is approxi-
mately 0.5Ω to 6Ω. This range is probably sufficiently wide
that most 2.2 µF Tantalum capacitors would fall within it.
At higher values of load currents (using a 2.2 µF output
capacitor), the stable ESR window gets very narrow. It is
likely that a Tantalum capacitor would not be a good choice
for a design that must be robust enough for mass production.
Instead, a 2.2 µF capacitor with very low ESR (either ce-
ramic or film) should be used with a 2Ω external resistor
placed in series to provide the ESR.
4.7 µF OUTPUT CAPACITOR
If a 4.7 µF capacitor is used, the stable range of ESR values
for 50 mA operation is approximately 0.6Ω to 6Ω. Because of
the reduced ESR values of the new Tantalum, it is possible
to find 4.7 µF Tantalum capacitors with ESR values at or
below 0.6Ω. To ensure a stable design, it is recommended
that an external resistor (value about 0.5Ω) be added in
series with the 4.7 µF Tantalum to provide adequate mini-
mum ESR.
At values of load current ≤ 20 mA, the ESR range is wide
enough that Tantalum can be used without external resis-
tance for added ESR.
Another acceptable configuration for 50 mA operation is to
use a ceramic or film 4.7 µF capacitor (which has very low
ESR) with an external 2Ω resistor in series.
10 µF OUTPUT CAPACITOR
50 mA operation using a 10 µF output capacitor requires an
ESR in the range of approximately 0.4Ω to 7Ω. As stated
previously, it is possible that solid Tantalum capacitors can
be found with ESR values near to or below 0.4Ω. An external
resistor in series with the Tantalum (value of about 0.5Ω) is
recommended to assure unconditional stability.
At values of load current ≤ 20 mA, the ESR range is wide
enough that Tantalum can be used without external resis-
tance for added ESR.
Another acceptable configuration for 50 mA operation is to
use a ceramic or film 10 µF capacitor (which has very low
ESR) with an external 2Ω resistor in series.
15 µF OUTPUT CAPACITOR
The stable ESR range (for 50 mA load current) using a 15 µF
output capacitor is approximately 0.1Ω to 10Ω. It is ex-
tremely unlikely that a 15 µF Tantalum capacitor would be
found with an ESR below 0.1Ω, so no external resistance is
required with a Tantalum.
As before, another acceptable configuration for 50 mA op-
eration is to use a ceramic or film 15 µF capacitor with an
external 2Ω resistor in series.
ON/OFF INPUT OPERATION
The LP2980 is shut off by pulling the ON/OFF input low, and
turned on by pulling it high. If this feature is not to be used,
this pin should be tied to V_IN to keep the regulator on at all
times.
To ensure proper operation, the signal source used to drive
the ON/OFF input must be able to swing above and below
the specified turn-on/turn-off voltage thresholds (see Electri-
cal Characteristics).
It is also important that the turn-on (and turn-off) voltage
signals applied to the ON/OFF 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 it is in the
range between the turn-on/turn-off voltage thresholds speci-
fied in the Electrical Characteristics section.
REVERSE INPUT/OUTPUT VOLTAGE
The PNP power transistor used as the pass element in the
LP2980 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 LP2980 to 0.3V (see Basic Ap-
plication Circuit).
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