CAT4002A, CAT4004A

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7

Basic Operation

The CAT400XA uses tightly matched current sinks to

accurately regulate LED current in each channel

proportional to the current sourced from the RSET pin.

There are 32 different settings for LED brightness that can

be programmed through the EN/DIM pin. Tight current

regulation for all channels is possible over a wide range of

input and LED voltages due to independent current sensing

circuitry on each channel.

Each LED channel needs a minimum of 50 mV headroom

to sink a constant regulated current of 20 mA. If the input

supply falls below 2.0 V typical, the under−voltage lockout

circuit disables all LED channels and resets the circuit to

default values. Any unused LED channels should be left

open.

LED Current Selection

After power−up, the LED current is set by the external

EN/DIM input as follows:

LED current + 125

0.6 V

31 * n

31

The full scale current is calculated from the above formula

with n equal to zero.

The EN/DIM pin has two primary functions. One function

enables and disables the device. The other function is LED

current dimming with 32 different levels by pulsing the input

signal, as shown on Figure 16. On each consecutive pulse

rising edge, the LED current is decreased by about 3.2%

current drops to zero, and then goes back to full scale on the

following pulse.

Each pulse width should be between 200 ns and 100

ms.

may shutdown the device. By pulsing the EN/DIM signal at

high frequency, the LED current can quickly be set to zero.

The LED driver enters a “zero current” shutdown mode if

EN/DIM is held low for 3 ms typical.

The dimming level is set by the number of pulses on the

EN/DIM after the power−up, as shown in Table 7.

Table 7. DIMMING LEVELS

Full Scale (Current in %)

Dimming Pulses [n]

100

0

97

1

94

2

90

3

87

4

84

5

81

6

77

7

74

8

71

9

68

10

65

11

61

12

58

13

55

14

52

15

48

16

45

17

42

18

39

19

35

20

32

21

29

22

26

23

23

24

19

25

16

26

13

27

10

28

6

29

3

30

0

31

100

32

Figure 16. EN/DIM Digital Dimming Timing Diagram

100%

EN/DIM

LED

Current

Shutdown

Shutdown

97% 94%

3%

0%

100%

32 Levels