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AAT3194

High Efficiency 1.5X Fractional Charge Pump For White LED Applications

Advanced Analogic Technologies 

PRODUCT DATASHEET

AAT3194

ChargePumpTM High Efficiency 1.5X Fractional Charge Pump For White LED Applications

700

600 VIN = 3.6V

500

400

300

200

100

0

0

20

40

60

80

100

Output (LED) Current (mA)

Figure 3: Input Power vs. LED Current.

Capacitor Selection

Careful selection of the four external capacitors CIN, C1,

C2, 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 gener-

al, 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 LED current sources are

only programmed for minimal 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

AAT3194. 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 maximize

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., greater than 2.2µF) are often

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

itors greater than 1µF are not typically required for

AAT3194 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.

Test Current/Channel Disable

Each channel of the output is equipped with a test cur-

rent function. The AAT3194 uses a small (~2µA) current

source injected into each output pin to detect the pres-

ence of an LED. Unused channels other than channel 1

should be left open and will be automatically disabled

instead of wasting the programmed output current.

Thermal Protection

The AAT3194 has a thermal protection circuit that will

shut down the charge pump and current outputs if the

die temperature rises above the thermal limit.

3194.2008.05.1.1

www.analogictech.com

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