AAT2806 datasheet(13/16 Pages) AAT

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Capacitor Characteristics

Ceramic composition capacitors are highly recom-

mended over all other types of capacitors for use with

the AAT2806. Ceramic capacitors offer many advan-

tages over their tantalum and aluminum electrolytic

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 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 important characteristic to consider when

selecting a capacitor. ESR is a resistance internal to

a capacitor that is caused by the leads, internal con-

nections, size or area, material composition, and

ambient temperature. Capacitor ESR is typically

measured 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 temperature. Larger capacitor val-

ues 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 capacitors

greater than 1µF are not typically required for

AAT2806 applications.

Capacitor area is another contributor to ESR.

Capacitors that are physically large will have a lower

ESR when compared to an equivalent material small-

er capacitor. These larger devices can improve cir-

cuit transient response when compared to an equal

value capacitor in a smaller package size.

Thermal Protection

The AAT2806 has a thermal protection circuit that

will shut down the two charge pumps if the die tem-

perature rises above the thermal limit.

Charge Pump Power Efficiency

Backlight Charge Pump: The charge pump effi-

ciency discussion in the following sections only

accounts for the efficiency of the charge pump sec-

tion itself. Due to the unique circuit architecture, it

is very difficult to measure efficiency in terms of a

percent value comparing input power over output

power. Since the outputs are pure constant current

sinks and typically drive individual loads, it is diffi-

cult to measure the output voltage for a given out-

put (D1 to D4) to derive an overall output power

measurement. For any given application, white

LED forward voltage levels can differ, yet the out-

put drive current will be maintained as a constant.

This makes quantifying output power a difficult task

when taken in the context of comparing to other white

LED driver circuit topologies. A better way to quantify

total device efficiency 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 applica-

tion should be based on trade-offs of size, external

components count, reliability, operating range, and

total energy usage...not just "% efficiency."

Efficiency of the AAT2806 may be quantified under

very specific conditions and is dependent upon the

input voltage versus the output voltage seen across

the loads applied to outputs D1 through D4 for a given

constant current setting. Depending on the combina-

tion of VIN and voltages sensed at the current sinks,

the device will operate in load switch mode. When

any one of the voltages sensed at the current sinks

nears dropout, the device will operate in 1.5X or 2X

charge pump mode. Each of these modes will yield

different efficiency values. Refer to the following two

sections for explanations for each operational mode.

Load Switch Mode Efficiency: The load switch

mode is operational at all times and functions alone

to enhance device power conversion efficiency when

VIN is greater than the voltage across the load. When

in load switch mode, the voltage conversion efficien-

cy is defined as output power divided by input power:

η =

POUT

AAT2806

Dual High Efficiency Charge Pump

for White LED and Flash Applications

2806.2006.04.1.8

AAT2806 Datasheet(PDF) 13 Page - Advanced Analog Technology, Inc.

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