Datasheet

LP2989LV
SNVS086J –MAY 2000–REVISED APRIL 2013 www.ti.com
The output capacitor must be located not more than 0.5" from the output pin and returned to a clean analog
ground.
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 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.
Capacitor Characteristics
Ceramic: The LP2989LV was designed to work with ceramic capacitors on the output to take advantage of the
benefits they offer: for capacitance values in the 10 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 10 µF
ceramic capacitor is in the range of 5 mΩ to 10 mΩ, which meets the ESR limits required for stability by the
LP2989LV.
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 10 µF Y5V capacitor were used on the output since it will drop down to
approximately 5 µF at high ambient temperatures (which could cause the LP2989LV 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.
For these reasons, X7R and X5R type ceramic capacitors must be used on the input and output of the
LP2989LV.
Tantalum: Tantalum output capacitors are not recommended for use with the LP2989LV because:
Tantalum capacitors are less desirable than ceramics for use as output capacitors because they are typically
more expensive when comparing equivalent capacitance and voltage ratings in the 1 µF to 10 µF range.
Another important consideration is that Tantalum capacitors have higher ESR values than equivalent size
ceramics. This means that while it may be possible to find a Tantalum capacitor with an ESR value within the
stable range, it would have to be larger in capacitance (which means bigger and more costly) than a ceramic
capacitor with the same ESR value.
Most 10 µF Tantalum capacitors have ESR values higher than the 0.5Ω maximum limit required to make the
LP2989LV stable.
It should also be noted that the ESR of a typical Tantalum will increase about 2:1 as the temperature goes from
25°C down to −40°C, so some guard band must be allowed.
Film: Polycarbonate and polypropelene film capacitors have excellent electrical performance: their ESR is the
lowest of the three types listed, their capacitance is very stable with temperature, and DC leakage currrent is
extremely low.
One disadvantage is that film capacitors are larger in physical size than ceramic or tantalum which makes film a
poor choice for either input or output capacitors.
However, their low leakage makes them a good choice for the noise bypass capacitor. Since the required
amount of capacitance is only .01 µF, small surface-mount film capacitors are avalable in this size.
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