cat4002a ON Semiconductor, cat4002a Datasheet - Page 7

• Manufacturer Part Number: cat4002a
• Description: Constant Current Led Driver With 32 Dimming Levels And Rset
• Manufacturer: ON Semiconductor

Basic Operation
accurately regulate LED current in each channel
proportional to the current sourced from the RSET pin.
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.
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
resistor (R
EN/DIM input as follows:
with n equal to zero.
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
current is 3.2% of the full scale current. On the 31
current drops to zero, and then goes back to full scale on the
following pulse.
Pulses faster than the minimum T
filtered by the device. Pulses longer than the maximum T
may shutdown the device. By pulsing the EN/DIM signal at
high frequency, the LED current can quickly be set to zero.
EN/DIM is held low for 3 ms typical.
EN/DIM after the power−up, as shown in Table 7.
The CAT400XA uses tightly matched current sinks to
There are 32 different settings for LED brightness that can
Each LED channel needs a minimum of 50 mV headroom
After power−up, the LED current is set by the external
The full scale current is calculated from the above formula
The EN/DIM pin has two primary functions. One function
Each pulse width should be between 200 ns and 100 ms.
The LED driver enters a “zero current” shutdown mode if
The dimming level is set by the number of pulses on the
th
of the full scale value). After 30 pulses, the LED
LED current + 125
SET
EN/DIM
Current
) value and the number of pulses (n) on the
LED
T
Shutdown
LED
0.6 V
R
SET
Figure 16. EN/DIM Digital Dimming Timing Diagram
LO
may be ignored and
T
100%
LO
31 * n
31
st
97% 94%
pulse, the
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T
HI
LO
32 Levels
7
Table 7. DIMMING LEVELS
Full Scale (Current in %)
3%
0%
100
100
97
94
90
87
84
81
77
74
71
68
65
61
58
55
52
48
45
42
39
35
32
29
26
23
19
16
13
10
6
3
0
100%
T
PWRDWN
Dimming Pulses [n]
Shutdown
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
0
1
2
3
4
5
6
7
8
9