AAT3193 datasheet(14/18 Pages) ANALOGICTECH

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AAT3193

3-Channel Charge-Pump LED Driver

ChargePumpTM

PRODUCT DATASHEET

3193.2008.07.1.3

www .analogictech.com

1μF

OUT

C-

C+

GND

RSET

EN/SET

14.3k

AAT3193

VIN

EN/SET

Figure 7: Lower Cost 1X Mode Application.

Capacitor Selection

Careful selection of the three external capacitors CIN, CP,

and COUT is important because they will affect turn-on

time, output ripple, and transient performance. Optimum

performance will be obtained when low equivalent series

resistance (ESR) ceramic capacitors are used; in general,

low ESR may be defined as less than 100mΩ. A value of

1μF for all four capacitors is a good starting point when

choosing capacitors. If the constant current sinks are

only programmed for light current levels, then the

capacitor size may be decreased.

Capacitor Characteristics

Ceramic composition capacitors are highly recommended

over all other types of capacitors for use with the

AAT3193. Ceramic capacitors offer many advantages

over their tantalum and aluminum electrolytic counter-

parts. 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 maximizing

charge pump transient response. Since ceramic capaci-

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

connection damage.

Equivalent Series Resistance

ESR is an important characteristic to consider when

selecting a capacitor. ESR is a resistance internal to a

capacitor that is caused by the leads, internal connec-

tions, size or area, material composition, and ambient

temperature. Capacitor ESR is typically measured in mil-

liohms for ceramic capacitors 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 gen-

erally have tight tolerance and are very stable over tem-

perature. Larger capacitor values are usually composed

of X7R, X5R, Z5U, or Y5V dielectric materials. Large

ceramic capacitors (i.e., larger than 2.2μF) are often

available in low cost Y5V and Z5U dielectrics, but capac-

itors larger than 1μF are not typically required for

AAT3193 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.

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