LP2989 Datasheet, PDF (18/34 Pages) National Semiconductor (TI) – Micropower/Low Noise, 500 mA Ultra Low-Dropout Regulator For Use with Ceramic Output Capacitors

LP2989
SNVS083O – FEBRUARY 2005 – REVISED MARCH 2015
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Figure 41. Stable Region for Output Capacitor ESR
NOTE
Important: The output capacitor must maintain its ESR within the stable region over the
full operating temperature range of the application to assure stability.
It is important to remember that capacitor tolerance and variation with temperature must be considered when
selecting an output capacitor so that the minimum required amount of output capacitance is provided over the full
operating temperature range. (See the Capacitor Characteristics section.)
The output capacitor must be located not more than 0.5 inches from the OUT pin and returned to a clean analog
ground.
8.2.2.2.3 Noise Bypass Capacitor
Connecting a 10-nF capacitor to the BYPASS pin significantly reduces noise on the regulator output. However,
the capacitor is connected directly to a high-impedance circuit in the bandgap reference.
Because this circuit has only a few microamperes flowing in it, any significant loading on this node will cause the
regulated output voltage to drop. For this reason, DC leakage current through the noise bypass capacitor must
never exceed 100 nA, and should be kept as low as possible for best output voltage accuracy.
The types of capacitors best suited for the noise bypass capacitor are ceramic and film. High-quality ceramic
capacitors with either NPO or COG dielectric typically have very low leakage. Ten-nF polypropolene and
polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low
leakage current.
8.2.2.3 Capacitor Characteristics
8.2.2.3.1 Ceramic
The LP2989 was designed to work with ceramic capacitors on the output to take advantage of the benefits they
offer: for capacitance values in the 4.7 µF range, ceramics are the least expensive and also have the lowest ESR
values (which makes them best for eliminating high-frequency noise). The ESR of a typical 4.7-µF ceramic
capacitor is in the range of 10 mΩ to 15 mΩ, which easily meets the ESR limits required for stability by the
LP2989.
One disadvantage of ceramic capacitors is that their capacitance can vary with temperature. Many large-value
ceramic capacitors (≥ 2.2 µF) are manufactured with the Z5U or Y5V temperature characteristic, which results in
the capacitance dropping by more than 50% as the temperature goes from 25°C to 85°C.
This could cause problems if a 4.7-µF capacitor were used on the output because it will drop down to
approximately 2.4 µF at high ambient temperatures (which could cause the LP2989 to oscillate). Another
significant problem with Z5U and Y5V dielectric devices is that the capacitance drops severely with applied
voltage. A typical Z5U or Y5V capacitor can lose 60% of its rated capacitance with half of the rated voltage
applied to it.
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