STEVAL-ILL013V1 Datasheet, PDF (17/43 Pages) STMicroelectronics – 80 W offline LED driver with PFC

Dimming function
8
Dimming function
UM0670
LEDs as a light source are very often used in applications where the brightness regulation is
required. Their biggest advantage is that their minimum brightness can be easily regulated
by changing their current, and they are stable even at very low brightness. Generally, there
are two basic concepts regarding how the brightness is regulated. The first is called “analog
dimming”, which means that the brightness is regulated by changing the continuous forward
LED current. This concept is not used on the STEVAL-ILL013V1. The second solution is to
use a low frequency (~200 Hz) PWM signal and change the brightness by pulse width
modulation. This is the approach used in the design of the STEVAL-ILL013V1. Any external
PWM generator can be used for brightness regulation, but it should be taken into account
that the STEVAL-ILL013V1 is not isolated.
In order to demonstrate the dimming function on the STEVAL-ILL013V1, an external PWM
generator using STMicroelectronics’ ST7LITEU05 microcontroller was connected to the
board, and the output LED current was measured. The microcontroller generates a PWM
signal with a frequency of 250 Hz. The duty cycle is set by a potentiometer from 0% up to
100%.The result with duty cycles of 50%, 10% and 2% is shown in Figure 12. The input
voltage was, in this case, 230 VAC and the output LED current was set to 350 mA. It is also
possible to achieve LED brightness regulation below 2%. In this case the nominal LED
current is slightly decreased.
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Doc ID 15327 Rev 2