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 (I
LED MAX
)
set by RSET and the PWM signal ON duty percentage.
I
LED
= I
LED 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
DUTY100%
DUTY50%
DUTY10%
DUTY25%
I
LED MAX
turn on
(I
LED
MAX
)
I
LED
= I
LED MAX
50%
I
LED
= I
LED MAX
10%
I
LED
= I
LED MAX
25%
I
LED
turn off
(0)
turn off
(0)
0
Dimming control with PWM signal
1period
ON
interval
ON
OFF
interval
10µsec
1period
100µsec
Duty [%] = ON interval/1period
OFF
I
LED
= I
LED
MAX
Duty [%]
PWM duty
SEIKO NPC CORPORATION —6
NPC [ NIPPON PRECISION CIRCUITS INC ]
