PROPRIETARY INFORMATION PRODUCT DATASHEET
AAT1271A
SwitchReg
TM
Functional Block Diagram
IN
1.5A Step-Up Current Regulator for Flash LEDs
SW
OUT
FLOUTA
FLOUTB
EN/SET
FLEN
FLINH
200kΩ
Control
RSET
CT
AGND PGND FLGND GND
Functional Description
The AAT1271A is a boost converter with a current regu-
lated output designed to drive high current white LEDs
used in camera flash applications. The AAT1271A has
two channels to accurately regulate the current flow
through two separate white LEDs. There are two basic
modes of operation in the AAT1271A; a Flash mode con-
trolled by the FLEN pin and the movie mode controlled
through the AS
2
Cwire interface.
The AAT1271A has two LED current sources which share
the output current equally. For a single white LED appli-
cation, the two current sources can be connected togeth-
er to apply full output current into the LED. In two LED
applications, each diode can be connected to its corre-
sponding current source (FLOUTA or FLOUTB) and the
output current will be shared. In applications where only
one LED is connected to either FLOUTA or FLOUTB, the
unused current sink must be directly connected to OUT,
thereby disabling that channel.
In mobile GSM systems where the phone remains in
constant contact with the base station by regular com-
munication, a FLINH pin is provided to prevent both the
camera flash and PA transmission pulses from occurring
simultaneously. This avoids potential dips to the Li-ion
battery voltage below the system’s undervoltage lockout
threshold (UVLO). During a flash event, strobing the
FLINH pin low-to-high reduces the LED current to the
default movie-mode current level for the duration of
FLINH. Strobing FLINH high-to-low instructs the
AAT1271A to revert the flash LED current to its maxi-
mum level, assuming that the FLEN pin is still active
(HIGH) and the flash timer has not expired.
Flash Mode
A flash pulse is initiated by strobing the FLEN input pin
low-to-high, which initiates a flash pulse and also starts
the internal timer. The maximum flash current in the
AAT1271A is set by an external resistor, R
SET
, which sets
the flash current and the maximum movie-mode current
reduced by a factor of 2. The flash timer will terminate
the flash current regardless of the status of the FLEN pin.
This can be either used as a simple flash timing pulse or
can be used as a safety timer in the event of a control
logic malfunction to prevent the LED from over-heating.
The maximum flash time is determined by an external
timing capacitor connected to the CT pin. The flash dura-
tion can be set from 50ms up to a maximum of 1s. The
AS
2
Cwire interface allows further adjustment of the flash
timer duration. This allows the flash timer duration to be
reduced in 16 linear steps from the maximum time set
by the timing capacitor. If the safety timer is not needed
in the application, it can be disabled by connecting the
CT pin directly to AGND.
Movie Mode
The movie mode current level, channel enable, and the
flash to maximum movie mode current ratio are set
using the AAT1271A’s AS
2
Cwire interface at the EN/SET
pin. The movie-mode current level can be adjusted in 16
steps using a logarithmic scale where each code is 1dB
below the previous code. Channel current outputs
(FLOUTA and FLOUTB) in movie-mode can be enabled or
1271A.2008.07.1.0
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