AAT2847_08 Datasheet, PDF (14/20 Pages) Advanced Analogic Technologies – Four-Channel Backlight Driver with Dual LDOs

ChargePump^TM
PRODUCT DATASHEET
AAT2847
Four-Channel Backlight Driver with Dual LDOs
Auto Disable Feature
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.
Low Dropout Regulators
The AAT2847 incorporates two LDO voltage regulators.
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 output (LDOA or LDOB) to the feedback
input (FBA or FBB). The ratio of the voltage divider resis-
tor values 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 dam-
age to the load or to the LDO.
Thermal Protection
The charge pump has built-in thermal protection cir-
cuitry 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.
Applications Information
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 forward voltage specifications
ranging from 2.0V to 4.7V. 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 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 (V_F) levels. Multiple
channels can be combined to obtain a higher LED drive
current without complication.
AAT2847-EE LDO Output
Voltage Programming
The output voltages for LDOA and LDOB are programmed
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 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
resistor values decrease efficiency.
LDO(A/B)
R2(A/B)
V_{LDO (A /B )}
FB(A/B)
V_REF = 1.2V
R1(A/B)
Figure 3: Selection of External Resistors.
To select appropriate resistor values, first choose a value
for R1 that will produce a reasonable feedback network
bias current. Then, according to the desired V_LDO(A/B), cal-
culate R2 according to the equation below. An example
calculation follows.
R1 is chosen to be 120kΩ, resulting in a small feedback
network bias current of 10µA (V_FB(A/B)/R1 = 1.2V/120kΩ).
The desired output voltage is 1.8V. From this informa-
tion, R2 is calculated from the equation below:
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2847.2008.05.1.2