AAT3153 datasheet(11/14 Pages) AAT

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AAT3153

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

For White LED Applications

3153.2005.12.0.93

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

component count, reliability, operating range, and

total energy usage...not just % efficiency.

The AAT3153 efficiency may be quantified under

very specific conditions and is dependent upon the

input voltage versus the output voltage seen across

the loads applied to inputs D1 through D6 for a

given constant current setting. Depending on the

combination of V

IN and voltages sensed at the cur-

rent 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 AAT3153 load switch mode is operational at all

times and functions alone to enhance device power

conversion efficiency when V

IN is greater than the

voltage across the load.

When in load switch

mode, the voltage conversion efficiency is defined

as output power divided by input power:

The expression to define the ideal efficiency (

η)

can be rewritten as:

-or-

Charge Pump Section Efficiency

The AAT3153 contains a fractional charge pump that

will boost the input supply voltage when V

IN is less

than the voltage required on the constant current

sink inputs. The efficiency (

η) can be simply defined

as a linear voltage regulator with an effective output

voltage that is equal to one and one half or two times

the input voltage. Efficiency (

η) for an ideal 1.5X

charge pump can typically be expressed as the out-

put power divided by the input power.

In addition, with an ideal 1.5X charge pump, the

output current may be expressed as 2/3 of the

input current. The expression to define the ideal

efficiency (

η) can be rewritten as:

-or-

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 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 light current lev-

els, 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 AAT3153. Ceramic capacitors offer many

advantages over their tantalum and aluminum elec-

η(%) = 100

OUT

1.5V

⎛⎞

⎝⎠

η =

OUT =

OUT

× I

OUT

× 1.5I

OUT

1.5V

η =

OUT

η(%) = 100

OUT

⎛⎞

⎝⎠

η =

OUT =

OUT

× I

OUT =

OUT

× I

OUT

η =

OUT

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