AAT2845
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 shut-
down mode, hold the EN/SET input low for longer
than TOFF (500µs). In this state, the AAT2845 typi-
cally 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 for-
ward voltage specifications ranging from 2.0V to
4.7V. LED applications may include mixed arrange-
ments 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 D1 to D4 constant current
sinks are matched with negligible voltage depend-
ence, the constant current channels will be matched
regardless of the specific LED forward voltage (VF)
levels.
LDO Output Voltage Programming
The output voltages for LDOA and LDOB are pro-
grammed 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 capaci-
tance to become a larger factor in circuit perform-
ance, whereas lower resistor values increase bias
current and decrease efficiency.
The low dropout current sinks in the AAT2845 max-
imize performance and make it capable of driving
LEDs with high forward voltages. Multiple channels
can be combined to obtain a higher LED drive cur-
rent without complication.
OUTA
(OUTB)
Device Switching Noise Performance
VOUT
The AAT2845 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 AAT2845 soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
R2
R1
FBA
(FBB)
VREF = 1.2V
Figure 2: Selection of External Resistors.
12
2845.2006.12.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
