Hi,
so it is ON for 387uS and off for 613uS.
That´s the point.
it is ON for 39% --> therefore brightness is about 39% of full brightness (which is continously ON)
(if a PWM is ON for 39% it is called 39%
duty cycle)
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Imagine it is ON for 613us and OFF for 387us then it is ON for 61% --> therefore brightness is about 61%
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The brightness can be controlled only by varying the voltage.
Here is your misunderstanding.
The brightness of a LED is almost proportional to the
current. Better say
average_current.
I recommend to look into a usual LED datasheet.
Here you will see that with 50% current (DC) the led brightness is about 50%, too. But the voltage is almost the same.
But varying the duty cycle from 0% to 100% will change the brightness from 0% to 100%.
But for sure the PWM frequency need to be high enough for your eyes not to recognize it.
Many years ago one said that 50Hz (20ms) is good. But it is only good for big areas, non moving areas and non moving eyes.
Therefore nowadays one uses 250Hz and more. But still with narrow points of light and fast moving one may recognize the PWM (frequency).
In your case it´s 1ms that means 1000Hz. Good frequency.
Klaus