AAT3131
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
such that outputs D1 to D3 can be controlled inde-
pendently of output D4. For Addresses 1 to 8, 9 to
16, 17 to 24, and 25 to 32, outputs D1 to D3 start
at 0mA and increase from 0.5mA to 20mA in three
8dB steps and three 2.5dB steps. Output D4
remains constant over these address ranges,
which provides orthogonal control of the two chan-
nels. For Addresses 1 to 8, D4 is set to 0mA; for
Addresses 9 to 16, D4 is set to 1mA; for Addresses
17 to 24, D4 is set to 15mA; and for Addresses 25
to 32, D4 is set to 30mA. This is summarized in
Table1 and Figure 1.
The modulo 32 interface wraps states back to State 1
after the 32nd clock. With each EN/SET pulse, the
output current changes to the next setting in the
address decoding. To change settings to the previous
address decoding, 31 EN/SET clock pulses are
required. The counter can be clocked at speeds up to
1MHz, so that intermediate states are not visible. The
first rising edge of EN/SET enables the IC and initial-
ly sets the output LED currents to 0mA. Additional
clocks are required to set the desired current level.
Once the final clock cycle is input for the desired
brightness level, the EN/SET pin is held high to main-
tain the device output current at the programmed
level. The device is disabled 500µs after the EN/SET
pin transitions to a logic low state.
Applications Information
Constant Current Output Level Settings
The constant current source output amplitude for
outputs D1 to D3 and D4 are set via the serial inter-
face according to a logarithmic scale. Using a log-
arithmic scale, LED brightness appears linear with
each increasing code count. Because the outputs
D1 to D4 are true independent constant current
sources, the voltage observed on any single given
output will be determined by the actual forward
voltage (V
F
) for the LED being driven.
Since the output current of the AAT3131 is program-
mable through its S
2
Cwire interface, no PWM (pulse
width modulation) or additional control circuitry are
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."
Furthermore, with its high-speed serial interface
(1MHz data rate), the output current of the AAT3131
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 programma-
bility and real-time control of LED brightness.
30
25
20
Current (mA)
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Address Code
Figure 1: Output Level Current Settings.
8
3131.2005.12.1.2
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
