AAT1270_08 Datasheet, PDF (10/18 Pages) Advanced Analogic Technologies – 1A Step-Up Current Regulator for Flash LEDs

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PRODUCT DATASHEET
AAT1270
1A Step-Up Current Regulator for Flash LEDs
The movie-mode output current is controlled by the EN/
SET pin, which uses a single-wire S²Cwire interface to
adjust the output current to one of 16 different levels. As
shown in Table 1, the current settings are in logarithmic
scale where each code is 1dB below the previous code. The
FLEN signal takes priority over movie-mode operation.
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 the cam-
era flash and PA transmission pulses from occurring
simultaneously, potentially causing the battery voltage
to drop below the system’s undervoltage lockout thresh-
old (UVLO) or creating unwanted EMI.
During a flash event, strobing the FLINH pin low-to-high
will reduce the LED current to the maximum movie mode
current level for the duration of FLINH. Strobing FLINH
high-to-low will instruct the AAT1270 to revert the flash
LED current to its default (maximum) level assuming
that the FLEN pin is still active (HIGH) and that the flash
timer has not expired.
Over-Temperature Protection
Thermal protection disables the AAT1270 when internal
power dissipation becomes excessive, as it disables both
MOSFETs. The junction over-temperature threshold is
140°C with 15°C of temperature hysteresis. The output
voltage automatically recovers when the over-tempera-
ture fault condition is removed.
Over-Voltage Protection
(Open LED, Open Circuit)
The AAT1270’s output voltage is limited by internal over-
voltage protection circuitry, which prevents damage to
the AAT1270 from open LED or open circuit conditions.
During an open circuit, the output voltage rises and
reaches 5.5V (typical), and the OVP circuit disables the
switching, preventing the output voltage from rising
higher. Once the open circuit condition is removed,
switching will resume. The controller will return to nor-
mal operation and maintain an average output voltage.
Applications Information
LED Selection
The AAT1270 is specifically designed to drive white flash
LEDs (typical forward voltage of 2.5V to 4.5V). Since the
FLOUTA and FLOUTB input current sinks are matched
with low voltage dependence; the LED-to-LED brightness
will be matched regardless of the individual LED forward
voltage (V_F) levels.
Flash Mode LED Current
The LED current is controlled by the R_SET resistor. For
maximum accuracy, a 1% tolerance resistor is recom-
mended. FLOUTA and FLOUTB can be programmed up to
a maximum total flash current of 1A, or up to 500mA per
channel. FLOUTA and FLOUTB output current is matched
across the programming range.
The maximum flash current in each FLOUTA and FLOUTB
is set by the R_SET resistor and can be calculated using the
following equation:
I_FLOUTA = I_FLOUTB =
81kΩ ⋅
R_SET
A
=
81kΩ ⋅ A
162kΩ
= 500mA per channel
Movie Mode LED Current
In the default movie mode, the corresponding FLOUTA/
FLOUTB movie mode current can be calculated:
I_{MOVIE MODE[A/B]}
=
I_{FLOUT[A/B]MAX}
7.3
For example, if an R_SET value of 162kΩ is chosen, then
the FLOUTA/FLOUTB flash current is set to 500mA. For
movie mode operation, the maximum current available
at either FLOUTA or FLOUTB is then:
I_{MOVIE MODE[A/B]}
=
I_{FLOUT[A/B]MAX}
7.3
=
500mA
7.3
= 68.5mA
10
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1270.2008.07.1.5