RT9166-12C View Datasheet(PDF) - Unspecified

Part Name

Description

Manufacturer

RT9166-12C

300/600mA, Ultra-Fast Transient Response LDO Regulator

Unspecified 

Preliminary

RT9166/A

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.

Ceramic:

For values of capacitance in the 10µF to 100µF range,

ceramics are usually larger and more costly than

tantalums but give superior AC performance for by-

passing high frequency noise because of very low ESR

(typically less than 10mΩ). 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.

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 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).

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 120Hz, which

indicates they have poor high frequency performance.

Only aluminum electrolytics that have an impedance

specified at a higher frequency (between 20kHz and

100kHz) should be used for the device. 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.

DS9166/A-09 October 2004

www.richtek.com

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