SM8136A

SEIKO NPC CORPORATION —5

FUNCTIONAL DESCRIPTION

LED Drive Current Setting

The SM8136A LED drive current can be set using the EN pin. The EN input is used to enable/disable circuit

operation. It also is used by a built-in counter that counts the number of input pulses in order to set the LED

current value. The built-in counter is a 4-bit counter for 16-step adjustment (DATA = 1 to 16). On the 16th

pulse (DATA = 16), the LED current drive circuit shuts down and LED current is zero. On the 17th pulse

(DATA = 1), the current returns to the maximum value. As the initial pulse (DATA = 1) sets the current to the

maximum value, the device can also be used in applications that do not support pulsed input.

EN Input Signal and LED Current Setting (Initialization)

On the first rising edge of EN, the internal startup signal goes HIGH and LED current will start to flow after the

soft start time has elapsed. As the LED current setting is adjusted by counting the number of input pulses on

EN, the EN input voltage must be switched HIGH

→ LOW → HIGH repeatedly to set the current to a lower

value. After the desired value is achieved, the EN input voltage is held HIGH to maintain the LED current set-

ting. If the EN input goes LOW, the LED current does not flow. The EN data input circuit operates even though

the LED current does not flow during the soft start interval, but as long as the current value is set during the soft

start interval the LED drive circuits will start to drive the LEDs with that constant current setting. However,

note that the setting is valid only after the power-ON reset when the VIN supply is applied. Accordingly, allow

1ms or more after connecting the VIN supply before setting the current value using the EN input.

I

LED current setting is determined by the number of EN input signal rising edges. Furthermore, no LED cur-

rent flows until the soft start time has elapsed.

Table 1. EN input pulse count and LED current setting values

DATA

123456789

10

11

12

13

14

15

16

EN

pulse

count

123456789

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

[mA]

20.00

18.7

17.3

16.0

14.7

13.3

12.0

10.7

9.3

8.0

6.7

5.3

4.0

2.7

1.3

0

EN input signal and LED current setting

EN input

signal

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8