SM8135B
(2) Setting LED current using pulse input on EN
The EN pin has a built-in counter that can be used to set the LED current to one of 14 levels (DATA1 to
DATA14) determined by the number of input pulses on EN. The LED current I
LED
begins to flow when EN is
first set to HIGH. The built-in counter is set to DATA1 (maximum setting), and the current is the maximum
LED current (I
LED MAX
) determined by resistor RSET. When an pulse input is applied on EN, the counter is
incremented and the current changes from I
LED MAX
by a factor of approximately 1/√2 for each pulse. The cur-
rent change is triggered by the falling edge of the input pulse, and each pulse should have a minimum 3.0µs
pulsewidth.
I
LED
= I
LED MAX
/(√2)
N
N: number of pulses = 0 to 13
On the 14th and 15th pulse (DATA15 and 16), the current drive circuit is shutdown and no LED drive current
flows, turning the LEDs OFF. On the 16th pulse, the current returns to the DATA1 maximum value. When the
EN input is held LOW for an interval longer than the EN hold time (1ms), the whole IC returns to standby
mode, and the LED current is set to 0. Note that the counter is also reset to value DATA1. Since the current is
set to the maximum value I
LED MAX
for 0 pulses (DATA1), applications that do not generate a pulse input can
still use the device to drive LEDs. In this case, the LED current setting is determined exclusively by RSET
(refer to section “(1) Setting LED drive current using RSET”).
DATA1
High
DATA2 DATA3 DATA4
DATA5
DATA15 DATA16
DATA1
EN hold time
EN
Low
turn on
(I
LED MAX
)
I
LED MAX
turn on
(I
LED MAX
)
1/ 2
1/ 2
1/ 2
I
LED
1/ 2
1/ 2
turn off
(0)
turn off turn off
(0)
(0)
turn off
(0)
0
Dimming control by EN pin
Table 1. EN input pulse count and I
LED
relationship (I
LED MAX
= 20mA)
DATA
1
0
DIM
pulse
count
16
32
48
I
LED
[mA]
Ratio
20.00
1
2
1
17
33
49
14.00
1/(√2)
1
3
2
18
34
50
10.00
1/(√2)
2
4
3
19
35
5
4
20
36
6
5
21
37
7
6
22
38
8
7
23
39
9
8
24
40
10
9
25
41
11
10
26
42
12
11
27
43
13
12
28
44
14
13
29
45
15
14
30
46
16
15
31
47
7.00
1/(√2)
3
5.00
1/(√2)
4
3.50
1/(√2)
5
2.50
1/(√2)
6
1.80
1/(√2)
7
1.27
1/(√2)
8
0.91
0.64
0.45
0.32
0.23
0
(OFF)
0
(OFF)
1/(√2)
9
1/(√2)
10
1/(√2)
11
1/(√2)
12
1/(√2)
13
SEIKO NPC CORPORATION —6
NPC [ NIPPON PRECISION CIRCUITS INC ]
