LM2991QML

SNVS392A – MARCH 2006 – REVISED OCTOBER 2011

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APPLICATION HINTS

EXTERNAL CAPACITORS

Like any low-dropout regulator, external capacitors are reqired to stabilize the control loop. These capacitors

must be correctly selected for proper performance.

INPUT CAPACITOR

An input capacitor is required if the regulator is located more than 6" from the input power supply filter capacitor

(or if no other input capacitor is present).

A solid Tantalum or ceramic capacitor whose value is at least 1 µF is recommended, but an aluminum electrolytic

(

≥ 10 µF) may be used. However, aluminum electrolytics should not be used in applications where the ambient

temperature can drop below 0°C because their internal impedance increases significantly at cold temperatures.

OUTPUT CAPACITOR

The output capacitor must meet the ESR limits shown in the graph, which means it must have an ESR between

about 25 m

Ω and 10Ω.

A solid Tantalum (value

≥ 1 µF) is the best choice for the output capacitor. An aluminum electrolytic (≥ 10 µF)

may be used if the ESR is in the stable range.

It should be noted that the ESR of a typical aluminum electrolytic will increase by as much as 50X as the

temperature is reduced from 25°C down to

−40°C, while a Tantalum will exhibit an ESR increase of about 2X

over the same range. For this and other reasons, aluminum electrolytics should not be used in applications

where low operating temperatures occur.

The lower stable ESR limit of 25 m

Ω means that ceramic capacitors can not be used directly on the output of an

LDO. A ceramic (

≥ 2.2 µF) can be used on the output if some external resistance is placed in series with it (1Ω

recommended). Dielectric types X7R or X5R must be used if the temperature range of the application varies

more than ± 25°C from ambient to assure the amount of capacitance is sufficient.

CERAMIC BYPASS CAPACITORS

Many designers place distributed ceramic capacitors whose value is in the range of 1000 pF to 0.1 µF at the

power input pins of the IC's across a circuit board. These can cause reduced phase margin or oscillations in LDO

regulators.

The advent of multi-layer boards with dedicated power and ground planes has removed the trace inductance that

(previously) provided the necessary "decoupling" to shield the output of the LDO from the effects of bypass

capacitors.

These capacitors should be avoided if possible, and kept as far away from the LDO output as is practical.

Figure 15. Output Capacitor ESR Range

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