LP3997 datasheet(10/11 Pages) NSC

10 / 11 page

Applications Information (Continued)

The ceramic capacitor’s capacitance can vary with tempera-

ture. The capacitor type X7R, which operates over a tem-

perature range of -55˚C to +125˚C, will only vary the capaci-

tance to within ±15%. The capacitor type X5R has a similar

tolerance over a reduced temperature range of -55˚C to

+85˚C. Most large value ceramic capacitors, larger than 1µF

are manufactured with Z5U or Y5V temperature characteris-

tics. Their capacitance can drop by more than 50% as the

temperature goes from 25˚C to 85˚C. Therefore X7R is

recommended over Z5U and Y5V in applications where the

ambient temperature will change significantly above or be-

low 25˚C.

Tantalum capacitors are less desirable than ceramic for use

as output capacitors because they are more expensive when

comparing equivalent capacitance and voltage ratings in the

1µF to 4.7µF 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. 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.

20092940

FIGURE 1. Graph Showing a Typical Variation in

Capacitance vs DC Bias

LP3997 Datenblatt(PDF) 10 Page - National Semiconductor (TI)