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Circuit Description
The FAN5702 is a white LED driver system based on an
adaptive 1.5x charge pump capable of supplying up to 180mA
of total output current. The tightly matched current sinks
ensure uniform brightness between the LEDs. Each LED has
a common anode configuration with its peak drive current set
Register Controlled Brightness
The DC value of the LED current is modulated according to
the values in Table 1. Current is expressed as a percentage of
the full scale current and is illustrated with a 20mA I SET .
during manufacturing ( see Ordering Information and I SET ). An
I 2 C-compatible interface is used to vary the brightness within
the individual current sinks as well as configure the grouping.
Table 1. Brightness Control
Dimming Code (Bx5-Bx0) Current Level
000000 0.125%
I LED (mA) (I SET =20mA)
0.025
Each LED is controlled with 64 exponentially spaced analog
brightness control levels through I 2 C, as indicated in Table 1.
For maximum flexibility, the FAN5702 can be programmed
with five independently controlled LED banks; by default,
arranged as 2,1,1,1,1 (first two LEDs represent Group A).
Through I 2 C, the device can be reconfigured to add up to six
LEDs to Group A as needed by application requirements.
Charge Pump
The charge pump operates in either 1x mode, where V OUT is
connected to V IN through a bypass switch, or in 1.5x mode.
The circuit operates in 1x mode until the LED with the highest
forward voltage (V LED(MAX) ) can no longer maintain current
regulation. At that point, 1.5x mode begins. If the lowest active
cathode voltage is greater than 1.8V, the charge pump
switches back to 1x mode.
IC Enable
By default the General Purpose register bit 7 = 0, the EN pin
functions as enable/disable. When the EN pin is LOW, all
circuit functions, including I 2 C, are disabled and the registers
are set to their default values.
When the EN pin HIGH, I 2 C interface is enabled. The LEDs
can be turned on/off by writing to the General Purpose
register. The user can always communicate via I 2 C with the
device to change register settings regardless of whether any
LED is on or off.
PWM Dimming
By programming the General Purpose register bit 7 = 1, the
EN pin is reappropriated to a PWM dimming input. Applying a
PWM signal to this pin controls the LED current waveform to
be ON when the PWM dimming pin is HIGH and OFF when
the PWM dimming pin is LOW. By using this pin in conjunction
with the I 2 C register dimming, the part can achieve higher
dimming resolution. For instance, an 8-bit PWM dimming
signal applied along with the 6-bit register dimming yields
better than 14 bits of resolution
To change the PWM dimming pin back to the EN function, set
the General Purpose register bit 7 to 0.
000001
000010
000011
000100
000101
000110
000111
001000
001001
001010
001011
001100
001101
001110
001111
010000
010001
010010
010011
010100
010101
010110
010111
011000
011001
011010
011011
011100
011101
011110
011111
100000
100001
100010
100011
100100
100101
100110
100111
101000
101001
101010
101011
101100
101101
101110
101111
110000
110001
110010
110011
110100
110101
110110
110111
111000
111001
111010
111011
111100
111101
111110
111111
0.188%
0.249%
0.312%
0.374%
0.438%
0.499%
0.560%
0.622%
0.692%
0.750%
0.810%
0.875%
0.938%
1.004%
1.124%
1.250%
1.375%
1.499%
1.625%
1.750%
1.881%
2.063%
2.249%
2.438%
2.687%
2.939%
3.186%
3.562%
3.936%
4.310%
4.813%
5.314%
5.936%
6.565%
7.313%
8.059%
8.938%
9.876%
10.874%
12.005%
13.253%
14.618%
16.124%
17.881%
19.875%
22.121%
24.621%
27.376%
30.373%
33.623%
37.124%
40.873%
44.875%
49.124%
53.624%
58.375%
63.378%
68.625%
74.122%
79.874%
85.873%
92.373%
100.000%
0.038
0.050
0.063
0.075
0.088
0.100
0.113
0.125
0.138
0.150
0.163
0.175
0.188
0.200
0.225
0.250
0.275
0.300
0.325
0.350
0.375
0.413
0.450
0.488
0.538
0.588
0.638
0.713
0.788
0.863
0.963
1.063
1.188
1.313
1.463
1.613
1.788
1.975
2.175
2.400
2.650
2.925
3.225
3.575
3.975
4.425
4.925
5.475
6.075
6.725
7.425
8.175
8.975
9.825
10.725
11.675
12.675
13.725
14.825
15.975
17.175
18.475
20.000
? 2010 Fairchild Semiconductor Corporation
FAN5702 ? Rev. 1.0.4
12
www.fairchildsemi.com
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