AAT3142 datasheet(14/16 Pages) AAT

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AAT3142

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

for White LED Applications

3142.2007.03.1.2

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 C

IN,

1, 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 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 AAT3142. Ceramic capacitors offer many advan-

tages over their tantalum and aluminum electrolytic

counterparts. A ceramic capacitor 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 have tight tolerance and are stable over

temperature.

Large capacitor values are typically

composed of X7R, X5R, Z5U, or Y5V dielectric mate-

rials. Large ceramic capacitors, greater than 2.2µF,

are often available in low-cost Y5V and Z5U

dielectrics, but capacitors greater than 1µF are usu-

ally not required for AAT3142 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.

Thermal Protection

The AAT3142 has a thermal protection circuit that will

shut down the charge pump if the die temperature

rises above the thermal limit, as is the case during a

short-circuit of the CP pin.

Charge Pump Compatibility

The three-output AAT3142 is pin-compatible with

the AAT3140, AAT3123, and AAT3113 in TSOPJW-

12 packages. The AAT3142 offers an improved

overall efficiency, wider operating range, and the

ability to drive high-V

F type LEDs at full current.

The AAT3142 is well suited for battery-powered

applications using single-cell lithium-ion/polymer

batteries and 3-series connected dry cells (3.6V).

η(%) = 100

OUT

1.5V

⎛⎞

⎝⎠

η =

OUT =

OUT

× I

OUT

× 1.5I

OUT

1.5V

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