PROPRIETARY INFORMATION PRODUCT DATASHEET
AAT1271A
TM
SwitchReg
1.5A Step-Up Current Regulator for Flash LEDs
AS2Cwire Control of Movie Mode
Operation and Flash Safety Timer
Applications Information
LED Selection
In the AAT1271A control of the movie mode operation
and flash timer is managed by the Advanced Simple
Serial Control (AS2Cwire) interface. AS2Cwire relies on
the number of rising edges of the EN/SET pin to address
and load the AAT1271A’s registers. As shown in Table 1,
Address 0 controls the Movie Current level as a percent-
age of the maximum movie mode current level. Address
1 controls the safety timer duration as a percentage of
the maximum value set by an external timing capacitor.
Address 2 enables the two LED channels independently
during movie mode. Finally, Address 3 sets the maxi-
mum possible current for movie mode operation. The
maximum movie mode current is set as a fraction of the
flash current with peak value of 1/2 and default value of
1/7.3. The last column in Table 1 shows the default val-
ues for each of the address registers.
The AAT1271A is specifically designed to drive white
flash LEDs (typical forward voltage of 2.5V to 4.0V).
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 (VF) levels.
Flash Mode LED Current
FLOUTA and FLOUTB can be programmed up to a maxi-
mum total flash current of 1.5A or up to 750mA per
channel. The output currents in FLOUTA and FLOUTB are
equal.
The maximum flash current in each FLOUTA and FLOUTB
is set by the RSET resistor. For the desired flash current in
each output, the resistor value can be calculated using
the following equation:
AS2Cwire Serial Interface
AS2Cwire latches data or address after the EN/SET pin
has been held high for longer than tLAT (500μs). Address
or data is differentiated by the number of EN/SET rising
edges. Since the data registers are 4 bits each, the dif-
ferentiating number of pulses is 24 or 16, so that Address
0 is signified by 17 rising edges, Address 1 by 18 rising
edges and Address 2 by 19 rising edges and Address 3
by 20 rising edges. Data is applied to any number of ris-
ing edges between 1 and 16, inclusive. A typical write
protocol is a burst of EN/SET rising edges, signifying a
particular address, followed by a pause with EN/SET
held high for the prescribed tLAT timeout period, a burst
of rising edges signifying data, and a tLAT timeout for the
data registers. Once an address is set, then multiple
writes to the corresponding data register are allowed.
Address 0 is the default address on the first rising edge
after the AAT1271A has been disabled.
81kΩ × A 81kΩ × A
IFLOUTA = IFLOUTB
=
=
= 750mA per channel
RSET
107kΩ
A flash event is initiated by asserting the FLEN pin. A
flash event is automatically terminated when FLEN is de-
asserted or if the safety timer terminates before the
FLEN pin is de-asserted. Any time that the FLINH pin is
asserted, the default movie-mode current level will
appear at both FLOUTA and FLOUTB. The default movie-
mode current level will be maintained on FLOUTA and
FLOUTB as long as the FLINH and FLEN pins are assert-
ed, and the safety timer continues to run.
In addition to setting the flash current via RSET, the flash
current can be changed after FLEN is asserted by pro-
gramming the movie mode current register with 16 dif-
ferent steps.
EN/SET
Rising Edges
Default
(No programming)
Address
Function
0
1
2
3
17
18
19
20
Movie Mode Current
Flash Safety Timer
Movie Mode Output Configuration
Flash/Movie Mode Current Ratio
100%
16/16
FLOUTA and FLOUTB ON
1/7.3
Table 1: AS2Cwire Serial Interface Addressing.
w w w . a n a l o g i c t e c h . c o m
1271A.2008.07.1.0
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