CAT3643
LED Current Selection
At power-up, the initial LED current is set to full scale
(100% brightness) by the external resistor RSET as
follows:
levels by pulsing the input signal, as shown on Figure 3.
On each consecutive pulse rising edge, the LED current
is divided by half to 50%, then 25%, 12.5%, 6.25% and
3.125% dimming levels.
0.6V
LED current =
132×
RSET
Pulses faster than the minimum TLO may be ignored
and filtered by the device. Pulses longer than the
maximum TLO may shutdown the device.
The EN/DIM pin has two primary functions. One
function enables and disables the device. The other
function is LED current dimming with six different
The LED driver enters a “zero current” shutdown mode
if EN/DIM is held low for 1.5ms or more.
T
T
T
HI
PWRDWN
SETUP
EN/DIM
T
LED
T
LO
100%
100%
50%
25%
12.5%
LED
Current
6.25%
3.12%
Shutdown
Shutdown
Figure 3. EN/DIM Digital Dimming Timing Diagram
Table 1. LED Current Dimming Levels
EN/DIM
RSET Gain
LED Current
# of Pulses *
132×0.6V RSET
66×0.6V RSET
EN = High
132
1st
2nd
3rd
4th
5th
6th
xth
66
33×0.6V RSET
33
16.5× 0.6V RSET
8.25× 0.6V RSET
4.125× 0.6V RSET
132×0.6V RSET
GAIN× 0.6V RSET
16.5
8.25
4.125
132
Device cycling through gain selection
* The gain is changed on the rising edges of the EN/DIM input.
© Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
11
Doc. No. MD-5022 Rev. E
CATALYST [ CATALYST SEMICONDUCTOR ]
