AAT2846_08 Datasheet, PDF (14/22 Pages) Advanced Analogic Technologies – High Current Charge Pump with Dual LDO for Backlight and Flash Applications

PRODUCT DATASHEET
AAT2846
ChargePump^TM High Current Charge Pump with Dual LDO for Backlight and Flash Applications
Flash/Movie Mode Operation
The flash LED current level is set via the serial interface
according to a logarithmic scale where each code is
1.2dB lower than the previous code as a percentage of
the maximum current set by an external resistor, R_FSET
from FSET to AGND. Flash LED current is set between
14% and 100% of the maximum current for both chan-
nels 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 channels can be connected in parallel to drive
the sum of the two channel currents into a single LED.
Flash Strobe Input
The AAT2846 features an active high flash enable input,
FEN, that overrides the AS²Cwire programming and
drives both flash outputs, FL1 and FL2, at the maximum
current set by the flash current setting resistor R_FSET.
This signal also triggers an internal protection timer.
When the FEN input is strobed low, the flash current is
turned off independent 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 maxi-
mum time set by the external C_T capacitor is such that
the flash LEDs remain within their power dissipation limit
at the maximum current set by R_FSET. 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 automati-
cally reset.
Flash Protection Timer
The AAT2846 includes a protection timer set by the
capacitor C_T connected from C_T to AGND. The timer is
used to avoid thermal issues with flash LEDs operating
above their continuous power rating should a data error
occur in the controller. The flash protection time, T_F, is
calculated by the following equation:
T_F = 10 · C_T
where T_F is in seconds and C_T is in µF.
For example: if C_T = 0.1µF:
T_F = 10 · 0.1µF
= 1s
Low Dropout Regulators
The AAT2846 includes two independent LDO linear regu-
lators. 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 voltage dividers from the output (OUTA
or OUTB) to the feedback input (FBA or FBB). The regu-
lator 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 current allows
the regulator to maintain output voltage regulation.
Each LDO regulator can supply up to 200mA continuous
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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2846.2008.03.1.1