AAT3156 datasheet(13/14 Pages) AAT

13 / 14 page

Capacitor Selection

Careful selection of the four external capacitors

IN, C1, C2, and COUT) is important because they

will affect turn-on time, output ripple, and transient

performance.

Optimum

performance

will

obtained when low Equivalent Series Resistance

(ESR) ceramic capacitors are used. In general, low

ESR may be defined as less than 100m

Ω. A capac-

itor value of 1µF for all four capacitors is a good

starting point when choosing capacitors. If the con-

stant current sinks are only programmed for light

current levels, then the capacitor size may be

decreased.

Capacitor Characteristics

Ceramic composition capacitors are highly recom-

mended over all other types of capacitors for use

with the AAT3156. Ceramic capacitors offer many

advantages over their tantalum and aluminum elec-

trolytic counterparts. A ceramic capacitor typically

has very low ESR, is lowest cost, has a smaller

PCB footprint, and is non-polarized. Low ESR

ceramic capacitors help to maximize charge pump

transient response. Since ceramic capacitors are

non-polarized, they are not prone to incorrect con-

nection damage.

Equivalent Series Resistance

ESR is an important characteristic to consider

when selecting a capacitor. ESR is a resistance

internal to a capacitor which is caused by the leads,

internal connections, size or area, material compo-

sition, and ambient temperature. Capacitor ESR is

typically measured in milliohms for ceramic capac-

itors and can range to more than several ohms for

tantalum or aluminum electrolytic capacitors.

Ceramic Capacitor Materials

Ceramic capacitors less than 0.1µF are typically

made from NPO or C0G materials. NPO and C0G

materials typically have tight tolerance and are sta-

ble over temperature. Larger capacitor values are

typically composed of X7R, X5R, Z5U, or Y5V

dielectric materials. Large ceramic capacitors, typ-

ically greater than 2.2µF, are often available in low-

cost Y5V and Z5U dielectrics, but capacitors

greater than 1µF are typically not required for

AAT3156 applications.

Capacitor area is another contributor to ESR.

Capacitors that are physically large will have a lower

ESR when compared to an equivalent material,

smaller capacitor. These larger devices can improve

circuit transient response when compared to an

equal value capacitor in a smaller package size.

AAT3156

High Efficiency 1X/1.5X/2X Charge Pump

For White LED Applications

3156.2005.09.1.2

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