AAT3131 datasheet(11/14 Pages) AAT

11 / 14 page

different efficiency values. Refer to the following

two sections for explanations for each operational

mode.

Load Switch Mode Efficiency

The AAT3131 load switch mode is operational at all

times and functions alone to enhance device power

conversion efficiency when V

IN is greater than volt-

age across the load connected to the constant cur-

rent source outputs. When in load switch mode,

the voltage conversion efficiency is defined as out-

put power divided by input power:

The expression to define the ideal efficiency (

η)

can be rewritten as:

-or-

Charge Pump Section Efficiency

The AAT3131 contains a fractional charge pump

which will boost the input supply voltage in the

event that V

IN is less than the voltage required on

the constant current source outputs. The efficien-

cy (

η) can be simply defined as a linear voltage

regulator with an effective output voltage that is

equal to one and one half times the input voltage.

Efficiency (

η) for an ideal 1.5X charge pump can

typically be expressed as the output 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-

For a charge pump with an output of 5V and a nom-

inal input of 3.5V, the theoretical efficiency is 95%.

Due to internal switching losses and IC quiescent

current consumption, the actual efficiency can be

measured at 93%. These figures are in close agree-

ment for output load conditions from 1mA to 100mA.

Efficiency will decrease as load current drops below

0.05mA, or when level of V

IN approaches VOUT.

Refer to the Typical Characteristics section of this

datasheet for measured plots of efficiency versus

input voltage and output load current for the given

charge pump output voltage options.

Capacitor Selection

Careful selection of the four external capacitors

IN, C1, C2, 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 AAT3131. 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.

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

AAT3131

High Efficiency 1X/1.5X Fractional

Charge Pump for White LED Applications

3131.2005.12.1.2

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