PRODUCT DATASHEET
AAT2861
TM
ChargePump
Backlight/Flash LED Driver and Multiple LDO Lighting Management Unit
imately 1mA steps while the flash LED current can be set
Functional Description
between 0 and 600mA in 40mA steps or 0 to 300mA in
20mA steps for two channel flash version. The backlight
LED currents match to within 3% while the flash LED
currents match to within 5%.
The AAT2861 is a highly integrated backlight and photo-
flash driver family with three LDO regulators. The charge
pump LED driver powers the backlight and flash LEDs
from the 2.7V to 5.5V input voltage. The LDO regulators
are powered from the same input and produce regulated
output voltage between 1.2V and 3.3V. Control of the
LEDs and the LDO output voltage is through an AS2Cwire
serial interface for easy programming.
To eliminate the latency of the AS2Cwire serial interface,
the flash LED is enabled through a dedicated input,
FL_EN. The AAT2861 also includes a safety timer that
prevents thermal overstress of the flash LED(s). This is
important because many flash LEDs operate for a brief
period beyond their steady-state operating limitations.
If the flash driving hardware and/or software fail to turn
the LED off, the safety timer insures that the LED or
other circuitry is not damaged.
LED Drivers
The AAT2861 drives six or seven backlight LEDs up to
31mA each and a total of 600mA flash current through
one or two channels. The LEDs are driven from a charge
pump to insure that constant current is maintained over
the entire battery voltage range. The charge pump auto-
matically switches from 1X, to 1.5X, to 2X modes and
back to maintain the LED current while minimizing
power loss for high efficiency. The charge pump operates
at the high 1MHz switching frequency, allowing the use
of small 1ꢀF ceramic capacitors.
Both the backlight and flash LED currents are pro-
grammed through the AS2Cwire serial interface as are
the backlight fade-in/fade-out timer and the flash safety
timer. See the “AS2Cwire Serial Interface Programming”
section of this datasheet for more information on setting
the LED currents.
LDO Regulators
Each individual LED is driven by a current sink to GND,
allowing individual current control with high accuracy
over a wide range of input voltages and LED forward
voltages while maintaining high efficiency.
The AAT2861 includes three low dropout (LDO) linear
regulators. These regulators are powered from the bat-
tery and produce a fixed output voltage set through the
AS2Cwire serial interface. The output voltage can be set
to one of 16 output voltages between 1.2V and 3.3V. The
LDO regulators can be turned on/off with the external
enable pins EN_LDOA and EN_LDOB/C. Additionally, the
AS2Cwire interface allows the LDO regulators to be
enabled independently when the EN_LDOA and EN_
LDOB/C pins are pulled high.
The charge pump is controlled by the voltage across the
LED current sinks. When any one of the active backlight
current sink/channel voltages drops below 200mV or the
flash current sink/channel drops below 450mV, the
charge pump goes to the next higher mode (from 1X to
1.5X or from 1.5X to 2X mode) to maintain sufficient
LED voltage for constant LED current. The AAT2861 con-
tinuously monitors the LED forward voltages and uses
the input voltage to determine when to reduce the
charge pump mode for better efficiency. There is also a
300mV mode-transition hysteresis that prevents the
charge pump from oscillating between modes.
The LDO regulators require only a small 2.2ꢀF ceramic
output capacitor for stability. If improved load transient
response is required, larger-valued capacitors can be
used without stability degradation.
AS2Cwire Serial Interface Protocol
LED Current Control
The AAT2861 is dynamically programmable by the
AS2Cwire single-wire interface. AS2Cwire records rising
edges detected at the EN/SET pin to address and load
the data registers. The timing diagram in Figure 1 shows
the typical transmission protocol.
Both the backlight and flash LED currents are controlled
through the AS2Cwire serial interface. The backlight LED
current can be set between 0.5mA and 31mA in approx-
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AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
