LP3852, LP3855

SNVS174I – FEBRUARY 2003 – REVISED FEBRUARY 2015

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10.2.2.3 Selecting A Capacitor

It is important to note that capacitance tolerance and variation with temperature must be taken into consideration

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 a capacitor's ESR change 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).

10.2.2.4 Capacitor Characteristics

10.2.2.4.1

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.

10.2.2.4.2

Tantalum

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

the ideal value required for loop compensation.

Tantalum capacitors also have good temperature stability: a good quality tantalum capacitor will typically show a

capacitance value that varies less than 10-15% across the full temperature range of 125°C to

−40°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).

10.2.2.4.3

Aluminum

This capacitor type offers the most capacitance for the money. The disadvantages are that they are larger in

physical size, not widely available in surface mount, and have poor AC performance (especially at higher

frequencies) due to higher ESR and ESL.

Compared by size, the ESR of an aluminum electrolytic is higher than either tantalum or ceramic, and it also

varies greatly with temperature. A typical aluminum electrolytic can exhibit an ESR increase of as much as 50X

when going from 25°C down to

−40°C.

It should also be noted that many aluminum electrolytics only specify impedance at a frequency of 120 Hz, which

indicates they have poor high frequency performance. Only aluminum electrolytics that have an impedance

specified at a higher frequency (between 20 kHz and 100 kHz) should be used for the LP385X. Derating must be

applied to the manufacturer's ESR specification, since it is typically only valid at room temperature.

Any applications using aluminum electrolytics should be thoroughly tested at the lowest ambient operating

temperature where ESR is maximum.

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