PRODUCT DATASHEET
AAT2845
TM
ChargePump
Four-Channel Backlight Driver with Dual LDOs
THI
TOFF
TLO
TLAT
EN/SET
1
2
n-1
n ≤ 16
0
n-1
Data Reg
0
Figure 1: S2Cwire Serial Interface Timing.
Shutdown
Applications Information
Since the sink switches are the only power returns for all
loads, there is no leakage current when all of the sink
switches are disabled. To activate the shutdown mode,
hold the EN/SET input low for longer than TOFF (500µs).
In this state, the AAT2845 typically draws less than 1µA
from the input. Data and address registers are reset to
0 in shutdown.
LED Selection
The AAT2845 is specifically intended for driving white
LEDs. However, the device design will allow the AAT2845
to drive most types of LEDs with forward voltage speci-
fications ranging from 2.0V to 4.7V. LED applications
may include mixed arrangements for display backlight-
ing, color (RGB) LEDs, infrared (IR) diodes, and any
other load needing a constant current source generated
from a varying input voltage. Since the D1 to D4 con-
stant current sinks are matched with negligible voltage
dependence, the constant current channels will be
matched regardless of the specific LED forward voltage
(VF) levels.
AAT2845-EE LDO Output
Voltage Programming
The output voltages for LDOA and LDOB are programmed
by an external resistor divider network. As shown below,
the selection of R1 and R2 is a straightforward matter.
R1 is chosen by considering the tradeoff between the
feedback network bias current and resistor value. Higher
resistor values allow stray capacitance to become a
larger factor in circuit performance, whereas lower resis-
tor values increase bias current and decrease efficiency.
The low dropout current sinks in the AAT2845 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
complication.
LDO(A/B)
VOUT(A/B)
Device Switching Noise Performance
R2(A/B)
R1(A/B)
The AAT2845 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 AAT2845 soft-start feature prevents noise
transient effects associated with inrush currents during
start-up of the charge pump circuit.
FB(A/B)
VREF = 1.2V
Figure 2: Selection of External Resistors.
To select appropriate resistor values, first choose R1
such that the feedback network bias current is reason-
able. Then, according to the desired VOUT, calculate R2
according to the equation below. An example calculation
follows.
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2845.2008.05.1.2
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