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AN47

更新时间： 2024-11-14 03:18:47

品牌	Logo	应用领域
捷特科 - ZETEX		/
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6页	224K
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Getting more out of the ZXLD1350 - dimming techniques

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AN47 数据手册

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

-------------------------

≈

[for 0<D

<1]

WPM

OUT

ADJ

ZXLD1350

PWM

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.

Issue 1 - August 2006

www.zetex.com

12 3 4 5 6

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