LP2986

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SNVS137I – MARCH 1999 – REVISED SEPTEMBER 2015

Figure 31. ESR Curves For 5-V Output

Figure 32. ESR Curves for 2.5-V Output

NOTE

The output capacitor must maintain its ESR in the stable region over the full operating

temperature range of the application to assure stability.

The minimum required amount of output capacitance is 4.7 µF. Output capacitor size can be increased without

limit.

It is important to remember that capacitor tolerance and variation with temperature must be taken into

consideration when selecting an output capacitor so that the minimum required amount of output capacitance is

provided over the full operating temperature range. A good tantalum capacitor will show very little variation with

temperature, but a ceramic may not be as good (see Capacitor Characteristics).

8.2.2.3 Capacitor Characteristics

8.2.2.3.1

Tantalum

The best choice for size, cost, and performance are solid tantalum capacitors. Available from many sources, their

typical ESR is very close to the ideal value required on the output of many LDO regulators.

Tantalums also have good temperature stability: a 4.7 µF was tested and showed only a 10% decline in

capacitance as the temperature was decreased from +125°C to

−40°C. The ESR increased only about 2:1 over

the same range of temperature.

However, it should be noted that the increasing ESR at lower temperatures present in all tantalums can cause

oscillations when marginal quality capacitors are used (where the ESR of the capacitor is near the upper limit of

the stability range at room temperature).

8.2.2.3.2

Ceramic

For a given amount of a capacitance, ceramics are usually larger and more costly than tantalums.

Be warned that the ESR of a ceramic capacitor can be low enough to cause instability: a 2.2-µF ceramic

capacitor was measured and found to have an ESR of about 15 m

Ω.

If a ceramic capacitor is to be used on the LP2986 output, a 1-

Ω resistor should be placed in series with the

capacitor to provide a minimum ESR for the regulator.

Another disadvantage of ceramic capacitors is that their capacitance varies a lot with temperature:

Large ceramic capacitors are typically manufactured with the Z5U temperature characteristic, which results in the

capacitance dropping by a 50% as the temperature goes from +25°C to 80°C.

8.2.2.3.3

Aluminum

The large physical size of aluminum electrolytics makes them unattractive for use with the LP2986. Their ESR

characteristics are also not well suited to the requirements of LDO regulators.

The ESR of an aluminum electrolytic is higher than a tantalum, and it also varies greatly with temperature.

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