AAT3141 datasheet(14/16 Pages) AAT

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AAT3141

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

for White LED Applications

3141.2007.03.1.3

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 AAT3141 contains a fractional charge pump

which will boost the input supply voltage when V

is less than the voltage required on the constant

current source outputs. 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 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 the 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, 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 AAT3141. Ceramic capacitors offer many

advantages over their tantalum and aluminum elec-

trolytic 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 con-

nection damage.

η =

OUT

η =

OUT =

OUT

× I

OUT =

OUT

× I

OUT

η(%) = 100

OUT

⎛⎞

⎝⎠

η =

OUT

η =

OUT =

OUT

× I

OUT

× 1.5I

OUT

1.5V

η(%) = 100

OUT

1.5V

⎛⎞

⎝⎠

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