AAT3141 datasheet(13/16 Pages) AAT

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AAT3141

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

for White LED Applications

3141.2007.03.1.3

LED Selection

The AAT3141 is specifically intended for driving

white LEDs. However, the device design will allow

the AAT3141 to drive most types of LEDs with for-

ward voltage specifications ranging from 1.0V to

4.3V. LED applications may include main and sub-

LCD display backlighting, camera photo-flash appli-

cations, color (RGB) LEDs, infrared (IR) diodes for

remotes, and other loads benefiting from a controlled

output current generated from a varying input volt-

age. Since the D1 to D4 output current sources are

matched with negligible voltage dependence, the

LED brightness will be matched regardless of the

specific LED forward voltage (V

F) levels.

In some instances (e.g., in high luminous output

applications such as photo flash), it may be neces-

sary to drive high-V

F type LEDs. The low-dropout

current sources in the AAT3141 make it capable of

driving LEDs with forward voltages as high as 4.3V

at full current from an input supply as low as 3.0V.

Outputs can be paralleled to drive high-current

LEDs without complication.

Termination of Unused Current Source

Outputs

If any outputs (D1 to D4) are not used, they should

be terminated by connecting the respective unused

output directly to ground. This is required to assure

correct charge pump mode operation. If a given

unused output is not terminated to ground, it will

appear to the respective output’s feedback control

as an infinite impedance load and the unused con-

stant current source will be set for a maximum volt-

age. Although no current flows from the unused

current source, the voltage sensed at that node

could force the charge pump control to activate the

charge pump when it is not necessary. To prevent

this effect, simply connect the unused constant cur-

rent source outputs to ground.

Device Switching Noise Performance

The AAT3141 operates at a fixed frequency of

approximately 1MHz to control noise and limit har-

monics that can interfere with the RF operation of

cellular telephone handsets or other communica-

tion devices. Back-injected noise appearing on the

input pin of the charge pump is 20mV peak-to-

peak, typically ten times less than inductor-based

DC/DC boost converter white LED backlight solu-

tions. The AAT3141 soft-start feature prevents

noise transient effects associated with inrush cur-

rents during start-up of the charge pump circuit.

Power Efficiency and Device Evaluation

The charge pump efficiency discussion in the follow-

ing sections only accounts for efficiency of the

charge pump section itself. Due to the unique circuit

architecture and design of the AAT3141, it is very dif-

ficult to measure efficiency in terms of a percent

value comparing input power over output power.

Since the AAT3141 outputs are pure constant cur-

rent sources and typically drive individual loads, it

is difficult to measure the output voltage for a given

output (D1 to D4) to derive an overall output power

measurement. For any given application, white

LED forward voltage levels can differ, yet the out-

put drive current will be maintained as a constant.

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 AAT3141 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 outputs D1 through D4 for a

given constant current setting. Depending upon the

case of V

IN being greater than the specific voltage

seen across the load on D1 through D4, the device

will operate in load switch mode. If the voltage seen

on the constant current source output is less than

IN, then the device will operate in 1.5X or 2X charge

pump mode. Each of these modes will yield differ-

ent efficiency values. Refer to the following two sec-

tions for explanations of each operational mode.

Load Switch Mode Efficiency

The AAT3141 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,

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