LP2985-N

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SNVS018V – MARCH 2000 – REVISED JUNE 2013

APPLICATION HINTS

EXTERNAL CAPACITORS

Like any low-dropout regulator, the LP2985-N requires external capacitors for regulator stability. These

capacitors must be correctly selected for good performance.

Input Capacitor

An input capacitor whose capacitance is

≥ 1 µF is required between the LP2985-N input and ground (the amount

of capacitance may be increased without limit).

This capacitor must be located a distance of not more than 1 cm from the input pin and returned to a clean

analog ground. Any good quality ceramic, tantalum, or film capacitor may be used at the input.

Important: Tantalum capacitors can suffer catastrophic failure due to surge current when connected to a low-

impedance source of power (like a battery or very large capacitor). If a Tantalum capacitor is used at the input, it

must be ensured by the manufacturer to have a surge current rating sufficient for the application.

There are no requirements for ESR on the input capacitor, but tolerance and temperature coefficient must be

considered when selecting the capacitor to ensure the capacitance will be

≥ 1 µF over the entire operating

temperature range.

Output Capacitor

The LP2985-N is designed specifically to work with ceramic output capacitors, utilizing circuitry which allows the

regulator to be stable across the entire range of output current with an output capacitor whose ESR is as low as

5 m

Ω. It may also be possible to use Tantalum or film capacitors at the output, but these are not as attractive for

reasons of size and cost (see CAPACITOR CHARACTERISTICS).

The output capacitor must meet the requirement for minimum amount of capacitance and also have an ESR

(equivalent series resistance) value which is within the stable range. Curves are provided which show the stable

ESR range as a function of load current (see Figure 36).

Figure 36. ESR Graph

Important: The output capacitor must maintain its ESR within the stable region over the full operating

temperature range of the application to assure stability.

The LP2985-N requires a minimum of 2.2 µF on the output (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. It should be noted that ceramic capacitors can exhibit large

changes in capacitance with temperature (see CAPACITOR CHARACTERISTICS). The output capacitor must be

located not more than 1 cm from the output pin and returned to a clean analog ground.

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