AAT1282 Datasheet, PDF (11/22 Pages) Skyworks Solutions Inc. – 2A Driver for High Intensity LED Camera Flash

DATA SHEET
AAT1282
2A Driver for High Intensity LED Camera Flash
Applications Information
LED Selection
The AAT1282 is specifically designed to drive ultra bright
flash LEDs with typical forward voltage of 2.5V to 4.0V
within 1A forward current. Since the FLOUTA and FLOUTB
pins have internal current-mirror circuitry which matches
both channels with tight accuracy up to 1A per channel;
the LED-to-LED brightness will be matched regardless of
the individual LED forward voltage (V_F). Circuit designers
should consult the LED supplier for LED specifications.
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 2000mA or up to
1000mA per channel. FLOUTA and FLOUTB output current
is matched across the programming range. A flash event
is initiated by asserting the FLEN pin. A flash event is
automatically terminated when FLEN is disabled or if the
safety timer terminates before the FLEN pin is disabled.
The maximum flash current in each FLOUTA and FLOUTB
is set by the R_SET resistor and can be calculated using the
following equation:
81kΩ · A 81kΩ · A
I_FLOUTA = I_FLOUTB = _RSET
=
= ~1000mA per channel
80.6kΩ
To prevent excessive power dissipation during higher
flash current operation, R_SET values smaller than 80.6kΩ
are not recommended.
Since the super capacitor is the only source for the LED
flash current, the duration of the flash is determined by
the energy stored in the super capacitor. During flash,
the energy of the super capacitor is discharged; at the
same time, the voltage of the super capacitor is
decreased. Once the super capacitor voltage is lowered
to a level (the minimum sink pin voltage + the LED
forward voltage), the flash is ended. With a fully charged
super capacitor in place, the flash for two 1A LEDs can
last for more than 500ms.
Movie Mode LED Current
The maximum movie-mode current level is set by the
maximum, programmed flash current reduced by the
programmed flash-to-movie-mode ratio in which the
default value is 7.3:
I = _{MOVIE-MODE[A/B]}
I _{= FLOUT[A/B](MAX)}
7.3
1000mA
7.3
= 137mA
To change the configuration or the settings, the AAT1282
can be programmed via the I²C interface. Triggering the
FLEN low to high will enable a flash event with the
maximum flash current set by the R_SET resistor or with
programmed flash current set via the I²C interface.
Concurrently, the flash timer is also initiated. All data
and register contents are cleared (reset to the default
value) after each flash event.
Flash Safety Timeout
The AAT1282 includes a timer circuit that enables the
flash current for a programmed period of time. This fea-
ture eliminates the need for an external, housekeeping
baseband controller to contain a safety delay routine. It
also serves as a protection feature to minimize thermal
issues with the flash LEDs in the event an external con-
troller’s flash software routine experiences hang-up or
freeze. The flash safety timeout, T can be calculated by
the following equation:
T = 13.5s/μF · C_T
Where T is in seconds and C_T is the capacitance of the
timer capacitor in μF.
For example, using a 74nF capacitor for C_T sets the flash
timeout to:
Flash Safety Timeout = 13.5s/μF · 0.074μF = 1s
The relationship between the flash safety timeout and
the capacitance of the timer capacitor is illustrated in
Figure 1.
1200
1000
800
600
400
200
0
0 10 20 30 40 50 60 70 80 90
C_T Capacitor (nF)
Figure 1: Flash Safety Timeout
vs. Timer Capacitor.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
11
202305C • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013