AAT1271A Datasheet, PDF (14/19 Pages) Advanced Analogic Technologies – 1.5A Step-Up Current Regulator for Flash LEDs

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PROPRIETARY INFORMATION PRODUCT DATASHEET
AAT1271A
1.5A Step-Up Current Regulator for Flash LEDs
Flash to Maximum Movie Mode
Current Ratio – Address 3
The maximum movie-mode current is a fixed ratio of the
flash current controlled by Address 3. The ratio may be
varied from 1:2 to OFF in 16 linear steps as shown in
Table 5. The default value for Address 3 is Data=4 and
represents a flash to maximum movie mode current
level of 1 to 7.3.
Data
1
2
3
4*
5
6
7
8
9
10
11
12
13
14
15
16
FL to MM Ratio
1/2
1/3.8
1/5.5
1/7.3
1/8.9
1/10.5
1/12.2
1/13.8
1/14.9
1/16.5
1/18
1/19.6
1/21.1
1/22.6
1/24
OFF
Table 5: Address 3, Flash/Movie
Mode Current Ratio.
The default maximum movie mode current can be calcu-
lated:
I_{MOVIE MODE[A/B]}
=
I_{FLOUT[A/B]MAX}
7.3
For example, if an R_SET value of 107kΩ is chosen, then
the FLOUTA/FLOUTB flash current is set to 750mA. 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
=
750mA
7.3
=
103mA
The maximum movie mode current level can be calcu-
lated using the following equation:
I_{MOVIEMODE[A/B]}
=
81kΩ ×
R_SET
A
=
FL
to
1
MM
Ratio ^{= Max Movie}
Mode Current
Shutdown
Since the sink switches are the only power returns for all
loads, there is no leakage current to load if all the sink
switches are disabled. When EN/SET pin is held low for
an amount of time greater than t_OFF (500μs), the
AAT1271A enters shutdown mode and draws less than
1μA from V_IN. All data and address registers are cleared
(reset to 0) during shutdown.
Selecting the Boost Inductor
The AAT1271A controller utilizes PWM control and the
switching frequency is fixed. To maintain 2MHz maximum
switching frequency and stable operation, a 1μH inductor
is recommended. Manufacturer’s specifications list both
the inductor DC current rating, which is a thermal limita-
tion, and peak inductor current rating, which is deter-
mined by the saturation characteristics. Measurements at
full load and high ambient temperature should be per-
formed to ensure that the inductor does not saturate or
exhibit excessive temperature rise.
The inductor (L) is selected to avoid saturation at mini-
mum input voltage and maximum output load conditions.
Worst-case peak current occurs at minimum input volt-
age (maximum duty cycle) and maximum load. 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 volt-
age. 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 tem-
perature rise, or thermal derating guidelines:
I_RMS
=
I_PEAK
3
For a given inductor type, smaller inductor size leads to
an increase in DCR winding resistance and, in most
cases, increased thermal impedance. Winding resistance
degrades boost converter efficiency and increases the
inductor’s operating temperature:
P_{LOSS(INDUCTOR)} = I^2RMS · DCR
*Denotes the default value.
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1271A.2008.07.1.0