AAT3113 datasheet(11/15 Pages) AAT

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AAT3113, AAT3114/14A

High Efficiency 1.5X Fractional Charge

Pumps For White LED Applications

3113.2006.07.1.9

current drops below 0.05mA or when the level of

VIN approaches VOUT.

Refer to the Typical

Characteristics section of this datasheet for meas-

ured plots of efficiency versus input voltage and

output load current for the given charge pump out-

put voltage options.

Power Efficiency and Device Evaluation

The charge pump efficiency discussion in the previ-

ous section only accounts for efficiency of the

charge pump section itself. Due to the unique cir-

cuit architecture and design of the AAT3113 and

AAT3114/14A, it is very difficult to measure efficien-

cy in terms of a percent value comparing input

power over output power. Since the device outputs

are pure constant current sources, it is difficult to

measure the output voltage for a given output (D1

to D6) to derive an output power measurement. For

any given application, white LED forward voltage

levels can differ, yet the output drive current will be

maintained as a constant. This makes quantifying

output power a difficult task when taken in the con-

text of comparing to other white LED driver circuit

topologies. A better way to quantify total device effi-

ciency is to observe the total input power to the

device for a given LED current drive level. The best

white LED driver for a given application should be

based on trade-offs of size, external components

count, reliability, operating range, and total energy

usage...not just "% efficiency."

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 per-

formance. 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 100m

Ω. 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 recom-

mended over all other types of capacitors for use

with the AAT3113 and AAT3114/14A products.

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 capacitors are non-polarized, they

are not prone to incorrect connection damage.

Equivalent Series Resistance: ESR is an impor-

tant characteristic to consider when selecting a

capacitor. ESR is a resistance internal to a capac-

itor that is caused by the leads, internal connec-

tions, size or area, material composition, and ambi-

ent temperature. Capacitor ESR is typically meas-

ured in milliohms 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 generally

have tight tolerance and are very stable over tem-

perature. Larger capacitor values are usually com-

posed of X7R, X5R, Z5U, or Y5V dielectric materi-

als. Large ceramic capacitors (i.e., greater than

2.2µF) are often available in low-cost Y5V and Z5U

dielectrics, but capacitors greater than 1µF are not

typically required for AAT3113 or AAT3114/14A

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.

AAT3113/AAT3114 Input Power

vs. LED Current

100

200

300

400

500

600

700

100

120

140

Output (LED) Current (mA)

VIN = 3.6V

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