LM2991 Datasheet, PDF (7/19 Pages) National Semiconductor (TI) – Negative Low Dropout Adjustable Regulator

LM2991
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APPLICATION HINTS
SNVS099H – MAY 1999 – REVISED JUNE 2013
External Capacitors
Like any low-dropout regulator, external capacitors are required 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 inches 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 electrolytic types 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 Figure 19, which means it must have an ESR between
about 25 mΩ and 10Ω.
Figure 19. Output Capacitor ESR Range
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 mΩ 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° 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 "de-coupling" 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.
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