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SM8135A

White LED Driver IC

Nippon Precision Circuits  

SM8135A

(2) Setting LED current using PWM signal input on EN

The EN input is used to enable/disable circuit operation, but it can also be switched at speed using a PWM sig-

nal input to set the average LED drive current value. When using PWM signal input on EN only for dimming

control, the average LED drive current is given by the multiplication of the maximum current value (ILED MAX)

set by RSET and the PWM signal ON duty percentage.

ILED = ILED MAX × ON Duty [%]

The PWM signal ON duty is defined in the following figures (“PWM duty”). The PWM signal operating fre-

quency range is 10kHz to 100kHz (10µs to 100µs per period), and the allowable duty range is 10 to 100%.

However, LED drive current that can be set by PWM signal has lower limit (= 2mA). When setting current to

2mA or less, even if the signal ON duty is within the allowable range, the current between pins varies widely.

High

EN

Low

ILED MAX

DUTY100%

turn on

(ILED MAX)

DUTY50%

DUTY10%

DUTY25%

ILED

turn off

(0)

0

ILED = ILED MAX 50%

ILED = ILED MAX 10%

ILED = ILED MAX 25%

turn off

(0)

Dimming control with PWM signal

1period

10µsec 1period 100µsec

ON

OFF

interval interval Duty [%] = ON interval/1period

ON

OFF

ILED = ILED MAX Duty [%]

PWM duty

SEIKO NPC CORPORATION —6

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