LP2982 Datasheet, PDF (17/28 Pages) National Semiconductor (TI) – Micropower 50 mA Ultra Low-Dropout Regulator

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LP2982
SNVS128K – MARCH 2000 – REVISED JUNE 2016
8.2.2.2 Capacitor Characteristics
8.2.2.2.1 Tantalum
Tantalum capacitors are the best choice for use with the LP2982. Most good quality tantalums can be used with
the LP2982, but check the manufacturer's data sheet to be sure the ESR is in range.
It is important to remember that ESR increases at lower temperatures and a capacitor that is near the upper limit
for stability at room temperature can cause instability when it gets cold.
In applications which must operate at very low temperatures, it may be necessary to parallel the output tantalum
capacitor with a ceramic capacitor to prevent the ESR from going up too high (see Ceramic for important
information on ceramic capacitors).
8.2.2.2.2 Ceramic
TI does not recommend use of ceramic capacitors at the output of the LP2982. This is because the ESR of a
ceramic can be low enough to go below the minimum stable value for the LP2982. A 2.2-μF ceramic was
measured and found to have an ESR of about 15 mΩ, which is low enough to cause oscillations.
If a ceramic capacitor is used on the output, a 1-Ω resistor must be placed in series with the capacitor.
8.2.2.2.3 Aluminum
Because of large physical size, aluminum electrolytics are not typically used with the LP2982. They must meet
the same ESR requirements over the operating temperature range, more difficult because of their steep increase
at cold temperature.
An aluminum electrolytic can exhibit an ESR increase of as much as 50× when going from 20°C to −40°C. Also,
some aluminum electrolytics are not operational below −25°C because the electrolyte can freeze.
8.2.2.3 Reverse Current Path
The internal PNP power transistor used as the pass element in the LP2982 has an inherent diode connected
between the regulator output and input. During normal operation (where the input voltage is higher than the
output) this diode is reverse biased (see Figure 38).
V_IN
V_OUT
PNP
GND
Figure 38. LP2982 Reverse Current Path
However, if the input voltage is more than a V_BE below the output voltage, 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
the V_IN pin and out the ground pin, which can damage the part.
The internal diode can also be turned on if the input voltage is abruptly stepped down to a voltage which is a V_BE
below the output voltage.
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