Data Sheet

WLMDU9456001JT / 172946001
MagI
3
C Power Module
LDHM - LED Step Down High Current Module
We-online.com Würth Elektronik eiSos GmbH & Co. KG – Data Sheet Rev. 1.0
© May 2016 22/35
The maximum frequency of the PWM dimming signal (f
DIM,max
) should not exceed, as rule of thumb,1/10 of the switching
frequency:
f

= 800kHz ⇒  f
,
= 80kHz
(
5
)
The minimum dimming on-time pulse (t
DIM,on_min
) is around 16µs. The frequency of the dimming signal must be adjusted
according to the required dimming contrast ratio (CR). The contrast ratio is defined as the number of steps between the
minimum (dark) and maximum (full light) brightness of a lighting source.
The higher the CR, the lower is the frequency, as described by the equation below:
f
,
=
1
t
,_
∙ CR
 (6)
Example 1. If a PWM signal is generated by a 8 bit timer of a microcontroller, 256 different steps are available (the
achievable contrast ratio would be 256:1). According to the equation (6), the following dimming frequency is calculated:
f

=
1
16μs ∙ 256
= 244Hz(7)
A PWM dimming frequency of 250Hz can be used. Varying the duty cycle of the PWM signal, 256 different level of
brightness of the LEDs can be achieved, as shown in the picture below:
16µs
32µs
48µs
f
DIM
= 250Hz
f
DIM
= 250Hz
f
DIM
= 250Hz
Darker LED
Brighter LED