8.2.2.2.1 Input Capacitor

An input capacitor whose value is at least 2.2 µF is required between the LP2989 input and ground (the amount of capacitance may be increased without limit).

Characterization testing performed on the LP2989 has shown that if the value of actual input capacitance drops below about 1.5 µF, an unstable operating condition may result. Therefore, the next larger standard size (2.2 µF) is specified as the minimum required input capacitance. Capacitor tolerance and temperature variation must be considered when selecting a capacitor (see Capacitor Characteristics section) to assure the minimum requirement of 1.5 µF is met over all operating conditions.

The input capacitor must be located at a distance of not more than 0.5 inches from the input pin and returned to a clean analog ground. Any good quality ceramic or tantalum may be used for this capacitor, assuming the minimum capacitance requirement is met.

8.2.2.2.2 Output Capacitor

The LP2989 requires a ceramic output capacitor whose value is at least 4.7 µF. The actual amount of capacitance on the output must never drop below about 3.5 µF or unstable operation may result. For this reason, capacitance tolerance and temperature characteristics must be considered when selecting an output capacitor.

The LP2989 is designed specifically to work with ceramic output capacitors, using circuitry which allows the regulator to be stable across the entire range of output current with an output capacitor whose ESR is as low as 4 mΩ. It may also be possible to use Tantalum or film capacitors at the output, but these are not as attractive for reasons of size and cost (see the Capacitor Characteristics section).

The output capacitor must meet the requirement for minimum amount of capacitance and also have an equivalent series resistance (ESR) value which is within the stable range. Curves are provided which show the stable ESR range as a function of load current (see Figure 41).

Figure 41. Stable Region for Output Capacitor ESR

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.

8.2.2.3.1 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.

For these reasons, X7R and X5R type ceramic capacitors must be used on the input and output of the LP2989.

8.2.2.3.2 Tantalum

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 4.7 µF range.

Another important consideration is that Tantalum capacitors have higher ESR values than equivalent size ceramics; 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.

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.

Tantalum capacitors may be used on the input as long as the requirement for minimum capacitance is met.

8.2.2.3.3 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 current 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. Because the required amount of capacitance is only 0.01 µF, small surface-mount film capacitors are available in this size.