AAT2848

High-Current Charge Pump For Backlight Display and Flash Applications

ChargePump

TM

PRODUCT DATASHEET

2848.2008.05.1.0

13

www .analogictech.com

Applications Information

LED Selection

The AAT2848 is specifically intended for driving white

LEDs. However, the device design will allow the AAT2848

to drive most types of LEDs. LED applications may include

mixed arrangements for display backlighting, color (RGB)

LEDs, infrared (IR) diodes and any other load needing a

constant current source generated from a varying input

voltage. Since the BL1 to BL4 and FL1 to FL2 constant cur-

rent sinks are matched with negligible voltage depen-

dence, the constant current channels will be matched

The low-dropout current sinks in the AAT2848 maximize

performance and make it capable of driving LEDs with

high forward voltages. Multiple channels can be com-

bined to obtain a higher LED drive current without com-

plication.

Device Switching Noise Performance

The AAT2848 operates at a fixed frequency of approxi-

mately 1MHz to control noise and limit harmonics that

can interfere with the RF operation of cellular telephone

handsets or other communication devices. Back-injected

noise appearing on the input pin of the charge pump is

20mV peak-to-peak, typically ten times less than induc-

tor-based DC/DC boost converter white LED backlight

solutions. The AAT2848 soft-start feature prevents noise

transient effects associated with in-rush currents during

the start up of the charge pump circuit.

Power Efficiency and Device Evaluation

Charge-pump efficiency discussion in the following sec-

tions accounts only for the efficiency of the charge pump

section itself. Due to the unique circuit architecture and

design of the AAT2848, it is very difficult to measure

efficiency in terms of a percent value comparing input

power over output power.

Since the AAT2848 outputs are pure constant current

sinks/channels and typically drive individual loads, it is

difficult to measure the output voltage for a given output

(BL1 to BL4 and FL1 to FL2) to derive an overall output

power measurement. For any given application, white

LED forward voltage levels can differ, yet the output

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 application should be

based on trade-offs of size, external component count,

reliability, operating range and total energy usage...not

just "% efficiency."

Device Power Efficiency

The AAT2848’s backlight charge pump conversion effi-

ciency is defined as the power delivered to the white LED

load divided by the input power:

η =

=

P

LEDs

P

IN

V

V

X = Number of Backlight White LEDs

The expression to define the estimated ideal backlight

efficiency (η) for the AAT2848 in 1x mode is as follows:

η =

=

P

LEDs

P

IN

V

V

η =

; X = 1, 2, 3 or 4 and I

η =

X · V

V

V

LEDX

V

IN

The AAT2848’s backlight charge pump is a fractional

charge pump which will boost the input supply voltage in

age across the backlight white LED load.

With an ideal 1.5x charge pump, the input current is

1.5x of the output current. The expression to define the

estimated ideal backlight efficiency (η) for the AAT2848

in 1.5x mode is as follows:

η =

=

P

LEDs

P

IN

V

V

η =

; X = 1, 2, 3 or 4 and I

η =

X · V

V

V

LEDX

1.5V

IN