AAT3122_05 ANALOGICTECH [Advanced Analogic Technologies], AAT3122_05 Datasheet - Page 8

• Manufacturer Part Number: AAT3122_05
• Description: High Efficiency 1X/1.5X Fractional Charge Pump for White LED Applications
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

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.
8
Code
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
9
Table 1: Current Level Settings.
I
OUT
10.8
11.8
13.6
15.1
16.9
18.8
3.3
3.8
4.2
4.7
5.2
6.1
6.6
7.5
8.5
9.4
(mA)
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
Code
4
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
5
Figure 1: Normalized Current Level Settings.
6
7
8
I
OUT
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
106.8
120.0
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
(mA)
High Efficiency 1X/1.5X Fractional Charge
Code
Pump for White LED Applications
EN/SET Serial Interface
The current source output magnitude is controlled
by the EN/SET serial 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
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.
AAT3122
The interface
3122.2005.11.1.3
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). During