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SM8135B

White LED Driver IC

Nippon Precision Circuits  

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 ILED 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 (ILED MAX) determined by resistor RSET. When an pulse input is applied on EN, the counter is

incremented and the current changes from ILED 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.

ILED = ILED 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 ILED 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”).

High

EN

Low

DATA1 DATA2 DATA3 DATA4 DATA5

ILED MAX

turn on

(ILED MAX)

1/ 2

DATA15 DATA16 DATA1

EN hold time

turn on

(ILED ) MAX

1/ 2

ILED

turn off

(0)

0

1/ 2

1/ 2

1/ 2

turn off turn off

(0)

(0)

turn off

(0)

Dimming control by EN pin

Table 1. EN input pulse count and ILED relationship (ILED MAX = 20mA)

DATA 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15

DIM 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

pulse

count 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

48 49 50

ILED

[mA]

Ratio

20.00 14.00 10.00 7.00 5.00 3.50 2.50 1.80 1.27 0.91 0.64 0.45 0.32 0.23 0

1 1/(√2)1 1/(√2)2 1/(√2)3 1/(√2)4 1/(√2)5 1/(√2)6 1/(√2)7 1/(√2)8 1/(√2)9 1/(√2)10 1/(√2)11 1/(√2)12 1/(√2)13 (OFF)

0

(OFF)

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