LP2989

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SNVS083O – FEBRUARY 2005 – REVISED MARCH 2015

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.

8.2.2.4 Reverse Input-Output Voltage

The PNP power transistor used as the pass element in the LP2989 has an inherent diode connected between

the regulator output and input.

During normal operation (where the input voltage is higher than the output) this diode is reverse-biased.

However, if the output is pulled above the input, this diode will turn on and current will flow into the regulator

output.

In any application where the output may be pulled above the input, an external Schottky diode must be

0.3 V (see the Absolute Maximum Ratings table).

8.2.3 Application Curves

Figure 43. Load Transient Response

Figure 42. Line Transient Response

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