AN47 ZETEX | Alldatasheet
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Issue 1 - August 2006 1 www.zetex.com © Zetex Semiconductors plc 2006 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 di scusses 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: IOUT
0.1 DPWM
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.
www.zetex.com 2 Issue 1 - August 2006 © Zetex Semiconductors plc 2006 If the PWM frequency is higher than approximately 10kHz and the duty cycle above the specified minimum value, the device will remain active an d the output will be continuous, with a nominal output current given by: IOUT For PWM dimming an input bipolar transistor with open collector output is recommended. This will ensure the 200mV input shutdown threshold is achieved. It is possible to PWM directly without a buffer tr ansistor. This must be done with caution. Doing this will overdrive the internal 1.25V reference. If a 2.5V input level is used at 100% PWM (DC) the output current into the LED will be 2X the normal current which may destroy the ZXLD1350. Overdriving with a 5V logic signal is very likely to damage the device as it exceeds the ADJ pin voltage rating. Soft start and decoupling capacitors Any extra capacitor on the ADJ pin will affect the leading and falling edge of the PWM signal. Take this into account as the rise time will be increased by approximately 0.5ms/nF. Compare this with a 100Hz PWM. 50% duty cycle T on and Toff are 5ms at 1% duty cycle T on is 0.1ms. 1nF on the ADJ pin will cause 0.5ms rise ti me which result in an error and limitation in dimming at low duty cycles.
Issue 1 - August 2006 3 www.zetex.com © Zetex Semiconductors plc 2006 DC voltage dimming The ADJ pin can be overdriven by an external DC voltage (V ADJ), as shown, in order to override the internal voltage reference and adjust the output current to a value above or below the nominal value. GND ZXLD1350ADJ GND DC The nominal output current is then given by: IOUT
0.08 V× ADJ
Note that 100% brightness setting corresponds to VADJ = VREF. If V IN is 2.5V max the R SENSE should be increased by 2X R S. This will slightly decrease the efficiency by 1 to 2% . The input impedance of the ADJ pin is 200k /H9024 ±20%. This may be factor if the DC voltage with a relatively high output resistance. IOUT vs VADJ (RS=300m/H9024) 100 150 200 250 300 350 0 0.25 0.5 0.75 1 1.25 V mA Series1 Figure 1 Typical output current versus ADJ voltage with RS = 300m/H9024
Issue 1 - August 2006 5 www.zetex.com © Zetex Semiconductors plc 2006 Resistor dimming By connecting a variable resistor between ADJ and GND, simple dimming can be achieved. Capacitor CADJ is optional for better AC mains interference and HF noise rejection. Recommend value of CADJ is 0.22/H9262F. GND ZXLD1350ADJ GND RADJ RS *CADJ The current output can be determined using the equation: IOUT
0.08 R S⁄() RADJ×
Note that continuous dimming is not possible with a resistor. At some point the shutdown threshold will be reached and the output current reduced to zero. This can occur below 300mV. Note that a 1M/H9024 resistor will load the VREF on the ADJ pin. The VREF will now be divided down by the nominal 200k V REF resistance and the 1M R ADJ. The nominal voltage will now be approximately 1V. RS will need to be adjusted to set the maximum current. The +/20% tolerance of the input resistance should also be understood. See table below: Table 1 RADJ k/H9024Rint nom. k/H9024 Rint min. k/H9024 Rint max. k/H9024 VADJ nominal % error from nominal Due to Rint min. % error from nominal Due to Rint max. 1000 200 160 240 1.041 3.4% -3.3% 500 200 160 240 0.892 6.1% -5.7% 200 200 160 240 0.625 11.1% -10.0% 100 200 160 240 0.416 15.4% -13.3%
-0.05 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 200 400 600 800 1000 IOUT AN47 www.zetex.com 6 Issue 1 - August 2006 © Zetex Semiconductors plc 2006 For international sales offices visit www.zetex.com/offices Zetex products are distributed worldwide. For details, see www.zetex.com/salesnetwork This publication is issued to provide outline information only whic h (unless agreed by the company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contact or be regarded as a representation relating to the products or services concerned. The company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. Europe Zetex GmbH Kustermann-park Balanstraße 59 D-81541 München Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 europe.sales@zetex.com Americas Zetex Inc
700 Veterans Memorial Highway
Hauppauge, NY 11788 USA Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 usa.sales@zetex.com Asia Pacific Zetex (Asia Ltd) 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 asia.sales@zetex.com Corporate Headquarters Zetex Semiconductors plc Zetex Technology Park, Chadderton Oldham, OL9 9LL United Kingdom Telephone: (44) 161 622 4444 Fax: (44) 161 622 4446 hq@zetex.com Figure 4 Typical output current against pot resistance If linear pot is used, the output current change is not linear against shaft rotation. In order to make the output current more linear, a log type pot is used. IOUT vs shaft rotation 100 150 200 250 300 350 0 1 02 03 04 05 06 07 08 09 0 1 0 0 Shaft rotation (%) IOUT (mA) Figure 5 Output current against shaft rotation of log type pot