AN47
Getting more out of the ZXLD1350 - dimming techniques
Ray Liu, Systems Engineer, Zetex Semiconductors
Introduction
The ZXLD1350 has a versatile adjust pin that can be used in many ways to adjust the brightness
of the LED by controlling the current in the LED. This application note deals with some the ways
in which dimming the LED can be achieved and discusses the merits of the techniques. These
dimming methods discussed include PWM dimming both with a low and high frequency signals,
DC voltage control and resistive dimming.
Low frequency dimming
Low frequency dimming is preferred for LED dimming since the LED instantaneous driving
current is constant. The color temperature of the LED is preserved at all dimming levels. Another
advantage of low frequency dimming is that the dimming level can down to 1%. Hence result in
dimming range of 100:1.
Choice of frequency
To avoid visible flicker the PWM signal must be greater than 100Hz. If you choose too high a
frequency the internal low pass filter will start to integrate the PWM signal and produce a non
linear response. Also the soft start function of the ADJ pin will cause a delay on the rising a falling
edge of the PWM signal. This can give a non-linearity in the LED current which will have a greater
affect as frequency increases.
An upper limit of 1kHz is suggested. The effect of audible noise in the inductor may need to be
considered. This may happen in some inductors with loose windings and will be more noticeable
at PWM frequencies of 1kHz than 100Hz.
If the PWM frequency is less than approximately 500Hz, the device will be gated 'on' and 'off' and
the output will be discontinuous, with an average value of output current given by:
0.1 D
PWM
-
I
OUT
≈
-------------------------
R
S
[for 0<D
WPM
<1]
ADJ
PWM
ZXLD1350
GND
GND
High frequency dimming
High frequency dimming is preferred if system required low radiated emission and in/output
ripple. But dimming range is reduced to 5:1. The ZXLD1350 has an internal low pass filter which
integrates the high frequency PWM signal to produce a DC dimming control.
© Zetex Semiconductors plc 2006
ZETEX [ ZETEX SEMICONDUCTORS ]
