AAT2848 Datasheet, PDF (13/19 Pages) Advanced Analogic Technologies – High-Current Charge Pump For Backlight Display and Flash Applications

ChargePump^TM
PRODUCT DATASHEET
AAT2848
High-Current Charge Pump For Backlight Display and Flash Applications
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
regardless of the specific LED forward voltage (V_F) levels.
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_LED1 ·
I_LED1 + ... + V_LEDX · I_LEDX
V_IN · I_IN
V_LEDx = White LED Forward Voltage (V_F)
I_LEDx = White LED Bias Current (I_D)
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_LED1 · I_LED1 + ... + V_LEDX
V_IN · I_IN
· I_LEDX
η
=
X
·
V_LEDX · I_LEDX
V_IN · I_IN
;
X
=
1,
2,
3
or
4
and
I_IN
=
X
·
I_LEDX
η
=
V_LEDX
V_IN
The AAT2848’s backlight charge pump is a fractional
charge pump which will boost the input supply voltage in
the event where V_IN is less then the required output volt-
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_LED1 · I_LED1 + ... + V_LEDX
V_IN · I_IN
· I_LEDX
η
=
X
·
V_LEDX · I_LEDX
V_IN · I_IN
;
X
=
1,
2,
3
or
4
and
I_IN
=
1.5(X
·
I_LEDX)
η
=
V_LEDX
1.5V_IN
2848.2008.05.1.0
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