AAT3122_05 Datasheet, PDF (8/12 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications

AAT3122
High Efficiency 1X/1.5X Fractional Charge
Pump for White LED Applications
Applications Information
Current Level Settings
LED current level is set via the serial interface
according to a logarithmic scale, where each code
is 1dB greater than the previous code. In this man-
ner, the LED brightness appears linear with each
increasing code. Table 1 depicts the relationship
between each rising edge of the EN/SET and the
output current in mA.
Code
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
I_OUT (mA)
3.3
3.8
4.2
4.7
5.2
6.1
6.6
7.5
8.5
9.4
10.8
11.8
13.6
15.1
16.9
18.8
Code
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
I_OUT (mA)
21.2
24.0
26.8
30.1
33.9
38.1
42.4
47.5
53.6
60.2
67.3
75.8
84.7
95.1
106.8
120.0
Table 1: Current Level Settings.
EN/SET Serial Interface
The current source output magnitude is controlled
by the EN/SET serial interface. The interface
records rising edges of the EN/SET pin and
decodes them into 32 individual current level set-
tings each 1dB apart. Code 32 is full scale, and
Code 1 is full scale attenuated by 31dB. The mod-
ulo 32 interface wraps states back to State 1 after
the 32nd clock, so 1dB of attenuation is achieved
by clocking the EN/SET pin 31 times. The counter
can be clocked at speeds up to 1MHz, so interme-
diate states are not visible. The first rising edge of
EN/SET enables the IC and initially sets the output
LED current to 3.3mA, the lowest setting. Once the
final clock cycle is input for the desired brightness
level, the EN/SET pin is held high to maintain the
device output current at the programmed level. The
device is disabled 500µs after the EN/SET pin tran-
sitions to a logic low state.
The EN/SET timing is designed to accommodate a
wide range of data rates. After the first rising edge
of EN/SET, the charge pump is enabled and reach-
es full capacity after the soft-start time (T_SS). During
the soft-start time, multiple clock pulses may be
entered on the EN/SET pin to set the final output
current level with a single burst of clocks.
Alternatively, the EN/SET clock pulses may be
entered one at a time to gradually increase the LED
brightness over any desired time period. A constant
current is sourced as long as EN/SET remains in a
logic high state. The current source outputs are
switched off after EN/SET has remained in a low
state for at least the T_OFF timeout period.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
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
Code
Figure 1: Normalized Current Level Settings.
8
3122.2005.11.1.3