LP2985-N_15 Datasheet, PDF (17/41 Pages) Texas Instruments – LP2985-N Micropower 150-mA Low-Noise Ultra-Low-Dropout Regulator in SOT-23 and DSBGA Packages Designed for Use with Very Low ESR Output Capacitors

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LP2985-N
SNVS018X – MARCH 2000 – REVISED MAY 2015
Figure 22. ESR Graph
NOTE
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.
9.2.2.1.3 Noise Bypass Capacitor
Connecting a 10 nF capacitor to the BYPASS pin significantly reduces noise on the regulator output. It should be
noted that 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 a
change in the regulated output voltage. 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. 10 nF polypropolene and
polycarbonate film capacitors are available in small surface-mount packages and typically have extremely low
leakage current.
9.2.2.2 Capacitor Characteristics
The LP2985-N was designed to work with ceramic capacitors on the output to take advantage of the benefits
they offer: for capacitance values in the 2.2 µF to 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 2.2
µF ceramic capacitor is in the range of 10 mΩ to 20 mΩ, which easily meets the ESR limits required for stability
by the LP2985-N.
One disadvantage of ceramic capacitors is that their capacitance can vary with temperature. Most 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 2.2 µF capacitor were used on the output since it will drop down to approximately
1 µF at high ambient temperatures (which could cause the LM2985 to oscillate). If Z5U or Y5V capacitors are
used on the output, a minimum capacitance value of 4.7 µF must be observed.
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