AAT3156
High Efficiency 1X/1.5X/2X Charge Pump
For White LED Applications
Since the constant current levels for the AAT3156 are
programmable, no PWM (pulse width modulation) or
additional control circuitry is needed to control LED
brightness. This feature greatly reduces the burden
on a microcontroller or system IC to manage LED or
display brightness, allowing the user to "set it and
forget it." With its high-speed serial interface (>1MHz
data rate), the LED current drive of the AAT3156 can
be changed successively to brighten or dim LEDs in
smooth transitions (e.g., to fade-out) or in abrupt
steps, giving the user complete programmability and
real-time control of LED brightness.
For each Max current scale, there are 16 current
level settings separated from one another by
approximately 1dB (see Current Settings table).
Code 1 is full-scale current and Code 15 is full-scale
attenuated roughly 14dB. Code 16 is reserved as a
"no current" setting.
AS
2
Cwire Serial Interface
The Advanced Simple Serial Control (AS
2
Cwire) sin-
gle wire interface is used to set the possible combi-
nations of current levels and LED channel states.
AS
2
Cwire has addressing capability for multiple data
registers. With multiple data registers, the AAT3156
main and sub-channels can be programmed togeth-
er or independently from one another.
AS
2
Cwire relies on the number of rising edges of the
EN/SET pin to address and load the registers.
AS
2
Cwire latches data or address after the EN/SET
pin has been held high for time T
LAT
. Address or data
is differentiated by the number of EN/SET rising
edges. Since the data registers are 4 bits each, the
differentiating number of pulses is 2
4
or 16, so that
Address 1 is signified by 17 rising edges, Address 2
by 18 rising edges, and so forth. Data is set to any
number of rising edges between 1 and including 16.
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 T
LAT
timeout
period, a burst of rising edges signifying Data, and a
T
LAT
timeout for the data registers. Once an address
is set, then multiple writes to the corresponding data
register are allowed.
When EN/SET is held low for an amount of time
greater than T
OFF
, the AAT3156 enters into shut-
down mode and draws less than 1µA from the sup-
ply. Address 1 is the default address on the first ris-
ing edge after the AAT3156 has been disabled.
Whenever shutdown mode is entered, all registers
are reset to 1.
Constant Current Programming Levels (mA)
1
Data
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
20mA Max
I
OUT
(mA)
20.0
17.8
15.9
14.3
12.7
11.1
10.2
8.9
7.9
7.0
6.3
5.7
5.1
4.4
4.1
0.0
2
30mA Max
I
OUT
(mA)
30.0
26.7
23.8
21.4
19.0
16.7
15.2
13.3
11.9
10.5
9.5
8.6
7.6
6.7
6.2
0.0
15mA Max
I
OUT
(mA)
15.0
13.3
11.9
10.7
9.5
8.3
7.6
6.7
6.0
5.2
4.8
4.3
3.8
3.3
3.1
0.0
1. There is an additional Low Current mode with currents down to 50µA. See the Low Current Register Settings section.
2. The device defaults to the 20mA Max scale. Use the Max Current Register to change the Max Scale.
3156.2005.06.1.1
9
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