Reference Design Note

MagI3C Power Modules

DNS005 MagI3C Multi Color LED Driver

DNS005 V1.0

Alexander Zeller

– June 2019

Page 4/20

Copyright © Würth Elektronik eiSos GmbH & Co. KG

3.1 Dimming with the LED Driver module

The LDHM (172946001) supports fast Pulse-Width Modulation (PWM) dimming of LEDs. An external microcontroller

controls the dimming of the LED driver by supplying the PWM signal to the DIM pin of the LED driver. This signal activates

and deactivates the MOSFET delivering power to the output of the LED driver. In addition, the dim-frequency depends on

the contrast ratio (CR) of the PWM signal. The CR defines the maximum dim-steps.

The PWM dimming is based on the refresh rate of the human eye. Therefore the current through the LED is periodically

interrupted with a frequency of 100 Hz to 1 kHz at the same amplitude. The longer the LED is interrupted, the less bright is

the intensity of the LED for the human eye. Due to the limitations of the human eye, humans will not see the flickering if the

dimming frequency is above 200 Hz. For camera applications, the frequency should be higher than 1 kHz to ensure the

dimming frequency is unobservable.

The advantage of this dimming method is that there are rarely differences in brightness or in color. This is because the

magnitude of the current through the LED is constant during the on time of the dimming cycle. Especially in the lower

percentages of the dimming value, this advantage benefits most.

There are important design considerations which have been taken into account when using this reference design due to

the use of PWM-controlled dimming. The frequency of the PWM signal can cause electromagnetic interference by

resonating with other elements of the system and potentially result in the manifestation of piezoelectric effects in the MLCCs.

Additionally, the PWM signal must have a controllable duty cycle to allow for manipulation of the dimming behavior.

Figure 5: Dimming with the LDHM

16µs

32µs

48µs

fDIM = 250Hz

fDIM = 250Hz

fDIM = 250Hz

Darker LED

Brighter LED