AAT3119 Datasheet, PDF(12/18 Page) Advanced Analogic Technologies

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AAT3119 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – High Efficiency 2X Charge Pump

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Flying Capacitor

Due to the switching operation of the voltage dou-

bling circuit topology, current flow through the flying

capacitor is bi-directional. The flying capacitor

selected must be a non-polarized type. A 1Î¼F low

ESR ceramic capacitor is ideal for this application.

Capacitor Characteristics

Ceramic composition capacitors are highly recom-

mended over all other types of capacitors for use with

the AAT3119. 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 impor-

tant characteristic to consider when selecting a

capacitor. ESR is a resistance internal to a capaci-

tor that is determined 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 0.1Î¼F are typically made from NPO or

C0G materials. NPO and C0G materials typically

have tight tolerance and are stable over tempera-

ture. Large capacitor values are typically com-

posed of X7R, X5R, Z5U, or Y5V dielectric materi-

als. Large ceramic capacitors, typically greater

than 2.2Î¼F, are often available in low-cost Y5V and

Z5U dielectrics, but large capacitors are not

required in most AAT3119 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 capac-

itor. These larger devices can improve circuit tran-

sient response when compared to an equal value

capacitor in a smaller package size.

AAT3119

High Efficiency 2X Charge Pump

Applications Information

White LED Backlight Driver

LED Selection: In applications where the AAT3119

is utilized as a white LED backlight driver, LEDs

with forward voltages up to 5.0V may be used. The

AAT3119 is available in two regulated output volt-

age versions: 4.5V and 5.0V. The output voltage

option selected will determine the maximum LED

forward voltage that can be driven. The trade-off

for the lower 4.5V output voltage version is the

deviceâs ability to supply greater output current.

Refer to the "Output Voltage vs. Output Current"

curves in the Typical Characteristics section of this

datasheet to determine the best AAT3119 output

voltage option based on the requirements of a

given application.

LED Ballast Resistors: To set the maximum

brightness of white LEDs connected in parallel from

a voltage source supply, a ballast resistor connect-

ed between each LED cathode and ground is

required. Refer to the application schematic in

Figure 1. The maximum brightness is determined

by the forward current (IF) through the respective

LED for a given forward voltage (VF). The typical

forward voltage of a specific LED is usually stated in

the typical characteristics of the given LED manu-

facturer's datasheet. The correct ballast resistor

value can be determined by the following equation:

RB =

(VOUT - VF)

Where:

RB =

VOUT =

VF =

Regulated charge pump output voltage

LED forward voltage at the desired

forward current

Desired LED forward current

3119.2007.02.1.2

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