AAT2846
High Current Charge Pump with Dual LDO
for Backlight and Flash Applications
centage of the maximum current set by an external
Flash Protection Timer
resistor, RFSET from FSET to AGND. Flash LED cur-
rent is set between 14% and 100% of the maximum
current for both channels through Address 4. For
lower current applications such as torch, flashlight,
or movie light, write to Address 5 to enable only FL1
or only FL2. For higher LED current, the flash chan-
nels can be connected in parallel to drive the sum of
the two channel currents into a single LED.
The AAT2846 includes a protection timer set by the
capacitor CT connected from CT to AGND. The
timer is used to avoid thermal issues with flash
LEDs operating above their continuous power rat-
ing should a data error occur in the controller. The
flash protection time, TF, is calculated by the fol-
lowing equation:
TF = 10 · CT
Flash Strobe Input
where TF is in seconds and CT is in µF.
For example: if CT = 0.1µF:
The AAT2846 features an active high flash enable
2
input, FEN, that overrides the AS Cwire program-
ming and drives both flash outputs, FL1 and FL2, at
the maximum current set by the flash current set-
ting resistor RFSET. This signal also triggers an
internal protection timer. When the FEN input is
strobed low, the flash current is turned off inde-
pendent of the internal timer. If FEN remains high
at the end of the protection timer period, the flash
LEDs are turned off. Make sure that the maximum
time set by the external CT capacitor is such that
the flash LEDs remain within their power dissipa-
TF = 10 · 0.1μF
= 1s
Low Dropout Regulators
The AAT2846 includes two independent LDO linear
regulators. The regulators operate from a 2.7V to
5.5V input voltage at IN. The AAT2846 supplies
separate LDO enable inputs (ENA and ENB) to
control individually the operation of the LDOs. The
LDO output voltages are set through resistive volt-
age dividers from the output (OUTA or OUTB) to
the feedback input (FBA or FBB). The regulator
controls the output voltage such that the voltage
divider output is at the 1.2V feedback threshold.
The low 200mV dropout voltage at 200mA load cur-
rent allows the regulator to maintain output voltage
regulation.
tion limit at the maximum current set by RFSET
.
Contact the LED manufacturer for details on the
thermal limitations of the LEDs used. When FEN is
used to control the flash LEDs, flash on/off control
set by Address 6 is automatically reset.
Each LDO regulator can supply up to 200mA con-
tinuous current to the load. They include current
limiting and thermal overload protection to prevent
damage to the load or to the LDOs.
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2846.2007.08.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
