AAT2847
Four-Channel Backlight Driver with Dual LDOs
Auto Disable Feature
Applications Information
The charge pump in the AAT2847 is equipped with
an auto-disable feature for each LED channel. After
the IC is enabled and successively starts-up, a test
current of 100µA (typical) is forced through each
LED channel. The channel will be disabled if the
voltage on that particular DX pin does not drop to
certain threshold. This feature is convenient for
disabling an unused channel or during an LED
short circuiting event.
LED Selection
The charge pump in the AAT2847 is specifically
intended for driving white LEDs. However, the
AAT2847 can 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
channels are matched with negligible voltage
dependence, the constant current channels will be
matched regardless of the specific LED forward
voltage (VF) levels. Multiple channels can be com-
bined to obtain a higher LED drive current without
complication.
Low Dropout Regulators
The AAT2847 incorporates two LDO voltage regu-
lators. The two regulators run from the same 2.7V
to 5.5V input voltage as the charge pump and have
separate ON/OFF control inputs, ENA and ENB.
For the AAT2847-EE, the LDO output voltages are
set through a resistive voltage divider from the out-
put (LDOA or LDOB) to the feedback input (FBA or
FBB). The ratio of the voltage divider resistor val-
ues determines the LDO output voltage. For the
AAT2847-QG option, LDOA is internally set to 2.8V
and LDOB is internally set to 1.5V. For the
AAT2847-QI option, LDOA is also 2.8V and LDOB
is internally set to 1.8V. Each LDO regulator can
supply a continuous load current up to 200mA, and
both LDOs include current limiting and thermal
overload protection to prevent damage to the load
or to the LDO.
AAT2847-EE LDO Output Voltage
Programming
The output voltages for LDOA and LDOB are pro-
grammed by an external resistor divider network.
As shown in Figure 3, the selection of R1 and R2 is
a straightforward matter. R1 is chosen by consider-
ing 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 resistor values
decrease efficiency.
Thermal Protection
LDO(A/B)
FB(A/B)
VLDO(A/B)
The charge pump has built-in thermal protection
circuitry that will shut down the charge pump and
the LDOs if the die temperature rises above the
thermal limit, as is the case during an OUT pin
short circuit event.
R2(A/B)
R1(A/B)
VREF = 1.2V
Figure 3: Selection of External Resistors.
To select appropriate resistor values, first choose a
value for R1 that will produce a reasonable feed-
back network bias current. Then, according to the
desired VLDO(A/B), calculate R2 according to the
equation below. An example calculation follows.
14
2847.2007.09.1.0
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