AAT3132ITP-1-T1(2004) データシートの表示（PDF） - Analog Technology Inc

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AAT3132ITP-1-T1
(Rev.:2004)

High Efficiency 1.5X Fractional Charge Pump for White LED Applications

Analog Technology Inc 

EN/SET Timing Diagram

EN/SET

Code OFF

AAT3132

High Efficiency 1.5X Fractional Charge

Pump for White LED Applications

tHI

tLO

tOFF

1

2

3

OFF

LED Selection

The AAT3132 is specifically intended for driving

white LEDs. However, the device design will allow

the AAT3132 to drive most types of LEDs with for-

ward voltage specifications ranging from 2.0V 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 con-

trolled output-current generated from a varying

input-voltage. 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 (VF)

levels.

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

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

sary to drive high-VF type LEDs. The low-dropout

current-sources in the AAT3132 makes 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 cur-

rent LEDs without complication.

Device Switching Noise Performance

The AAT3132 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 AAT3132 soft-start feature prevents

noise transient effects associated with in-rush cur-

rents during the start up of the charge pump circuit.

Power Efficiency and Device

Evaluation

The charge pump efficiency discussion in the fol-

lowing sections only account for the efficiency of the

charge pump section itself. Due to the unique circuit

architecture and design of the AAT3132, it is very

difficult to measure efficiency in terms of a percent

value comparing input power over output power.

Since the AAT3132 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 AAT3132 efficiency may be quantified under

very specific conditions and is dependant upon the

input voltage versus the output voltage seen

across the loads applied to outputs D1 through D4

for a given constant current setting. The efficiency

(η) can be simply defined as a linear voltage regu-

lator with an effective output voltage that is equal to

one and a 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.

10

3132.2004.04.1.0

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