AAT1270_08 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – 1A Step-Up Current Regulator for Flash LEDs

SwitchReg^TM
PRODUCT DATASHEET
AAT1270
1A Step-Up Current Regulator for Flash LEDs
The relationship between capacitance of the timer capac-
itor and the flash timeout is illustrated in Figure 2.
50
45
40
35
30
25
20
15
10
5
0
0 100 200 300 400 500 600 700 800 900 1000
Flash Timeout (ms)
Figure 2: Timer Capacitor vs. Flash Timeout.
Selecting the Boost Inductor
The AAT1270 controller utilizes PWM control and the
switching frequency is fixed. To maintain 2MHz maxi-
mum switching frequency and stable operation, an 1μH
output inductor is recommended.
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and peak
Timing Diagram
FLASH FLASH
FLASH
MOVIE MODE
LED CURRENT (0mA)
FLASH LOWERED
TO MOVIE-MODE
DURING FLINH
inductor current rating, which is determined by the satu-
ration characteristics. Measurements at full load and
high ambient temperature should be performed to
ensure that the inductor does not saturate or exhibit
excessive temperature rise.
The output inductor (L) is selected to avoid saturation at
minimum input voltage and maximum output load condi-
tions. Worst-case peak current occurs at minimum input
voltage (maximum duty cycle) and maximum load.
At high load the switching frequency is somewhat dimin-
ished, resulting in higher I_PEAK. Bench measurements are
recommended to confirm actual I_PEAK and to ensure that
the inductor does not saturate at maximum LED current
and minimum input supply voltage.
The RMS current flowing through the boost inductor is
equal to the DC plus AC ripple components. Under worst-
case RMS conditions, the current waveform is critically
continuous. The resulting RMS calculation yields worst-
case inductor loss. The RMS current value should be
compared against the inductor manufacturer’s tempera-
ture rise, or thermal derating, guidelines:
I_RMS
=
I_PEAK
3
FLASH
FLASH
tDLY(CT)
FLASH TIMES OUT
tDLY(CT)
FLASH TIMES OUT
SET BY CT
FLASH LOWERED
TO MOVIE-MODE
DURING FLINH
EN/SET
DATA TO
MOVIE-MODE
LEVEL
FLASH ENABLE
FLASH INHIBIT
12
www.analogictech.com
1270.2008.07.1.5