UM0670 datasheet(17/43 Pages) STMICROELECTRONICS

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Dimming function

UM0670

17/43

Doc ID 15327 Rev 2

Dimming function

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.

UM0670 датащи(PDF) 17 Page - STMicroelectronics