PNP

GND

17

LP2982

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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).

Figure 38. LP2982 Reverse Current Path

flow into the regulator output. In such cases, a parasitic SCR can latch which will allow a high current to flow into

below the output voltage.