LOAD CURRENT (A)

STABLE REGION

0

1

2

.001

.01

0.1

1.0

10

LP3872, LP3875

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SNVS227H – FEBRUARY 2003 – REVISED JANUARY 2015

Figure 14. ESR Curve

8.2.2.3 Selecting a Capacitor

Capacitance tolerance and variation with temperature must be considered when selecting a capacitor so that the

minimum required amount of capacitance is provided over the full operating temperature range. In general, a

good tantalum capacitor will show very little capacitance variation with temperature, but a ceramic may not be as

good (depending on dielectric type). Aluminum electrolytics also typically have large temperature variation of

capacitance value.

Equally important to consider is how ESR of a capacitor changes with temperature: this is not an issue with

ceramics, as their ESR is extremely low. However, it is very important in tantalum and aluminum electrolytic

capacitors. Both show increasing ESR at colder temperatures, but the increase in aluminum electrolytic

capacitors is so severe they may not be feasible for some applications (see Capacitor Characteristics).

8.2.2.4 Capacitor Characteristics

8.2.2.4.1

Ceramic

For values of capacitance in the 10-µF to 100-µF range, ceramics are usually larger and more costly than

tantalums but give superior AC performance for bypassing high frequency noise because of very low ESR

(typically less than 10 m

Ω). However, some dielectric types do not have good capacitance characteristics as a

function of voltage and temperature.

Z5U and Y5V dielectric ceramics have capacitance that 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. The Z5U and Y5V

also exhibit a severe temperature effect, losing more than 50% of nominal capacitance at high and low limits of

the temperature range.

X7R and X5R dielectric ceramic capacitors are strongly recommended if ceramics are used, as they typically

maintain a capacitance range within ±20% of nominal over full operating ratings of temperature and voltage. Of

course, they are typically larger and more costly than Z5U/Y5U types for a given voltage and capacitance.

8.2.2.4.2

Tantalum

Solid tantalum capacitors are recommended for use on the output because their typical ESR is very close to the

ideal value required for loop compensation. They also work well as input capacitors if selected to meet the ESR

requirements previously listed.

Tantalums also have good temperature stability: a good quality tantalum will typically show a capacitance value

that varies less than 10-15% across the full temperature range of

−40°C to 125°C. ESR will vary only about 2X

going from the high to low temperature limits.

The increasing ESR at lower temperatures can cause oscillations when marginal quality capacitors are used (if

the ESR of the capacitor is near the upper limit of the stability range at room temperature).

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