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
resistor (R
SET
) value and the number of pulses (n) on the
EN/DIM input as follows:
LED current + 125
0.6 V
R
SET
ǒ
31 * n
31
Ǔ
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%
(1/31
th
of the full scale value). After 30 pulses, the LED
current is 3.2% of the full scale current. On the 31
st
pulse, the
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.
Pulses faster than the minimum T
LO
may be ignored and
filtered by the device. Pulses longer than the maximum T
LO
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
T
LED
T
LO
T
HI
T
PWRDWN
