DN05065/D Design Note – DN05065/D Step Dimming Guidelines for NCL30081/83 LED Drivers Device Application Input Voltage Output Power Topology I/O Isolation NCL30081/83 LED Lighting 100 - 265 V ac na CrM na reducing the output current. Overview The ability to dim household lighting is a desirable feature that has been common in residential lighting for over 50 years. A dimmer accomplishes this is but requires installation replacing a switch in the appropriate electrical Dimming Table 1 box. The installation is often beyond the typical The internal circuit architecture is shown below home repair requiring a qualified electrician. The for the BO detection. NCL30081/83 incorporates a step dimming feature that changes the LED bulb output whenever the AC is interrupted briefly using a existing wall switch. Toggling the wall switch on/off for a 1-5 s steps down the bulb output in 6 The Vcc voltage will drop when the BO detector discrete steps. After the last step, the bulb goes shuts off the output. When Vcc drops to the back to 100% brightness. This approach allows Vcc_off threshold (~9 V), the current draw on the the customer to add dimming capability to a Vcc pin drops to about 50 µA. The state of the fixture without the need to make changes to the dimming register is retained until Vcc reaches electrical wiring. Vcc_reset which is about 5 V. When AC power returns, the BO detector senses good power and Theory of Operation turns on the output again at the new current The NCL30081/83 detects the presence of the AC setting. The size of the Vcc capacitor determines line through the Vin pin. If Vin is lost for more the hold up time between Vcc_off and Vcc_reset. than 50ms, the Brown Out detector turns off the This is the window of time to toggle the dimming output and decrements the step dimming register register. If Vcc drops below Vcc_reset, the August 2014, Rev. 0 www.onsemi.com 1 DN05065/D dimming register is cleared. When power is The decay of the Vcc voltage is dependent on Icc restored, the output will be at 100%. and the available stored energy in the Vcc filter circuit. If the Vcc voltage falls below Vcc_off before the BO timer has timed to 50ms, the Design Guidelines dimming register will not decrement the current. Use of a split Vcc supply is necessary to ensure It will hold the previous state. The result will be a adequate hold up time to provide 1s - 5s of step driver that fails to decrement the current when the dimming time while not inhibiting fast start up. input power is toggled. The NCL30081/83 The split supply is shown below. accepts a wide range of Vcc up to 26V while operating down to about 10V. Therefore it is advisable to operate Vcc as close to 26V as practical without causing an OVP condition. Setting Vcc can be a problem for designs that have wide output voltage requirements. One must also consider that the LED voltage will decrease some with reduced current. The driver may be able to drive a wide output voltage range but the step dimming range may be narrower because the Vcc decays before the BO timer decrements the The 47uF capacitor does not load the Vcc dimming current state. terminal during start up thus assuring a fast start. After the converter starts switching, the 47uF Output Voltage Rise time capacitor provides a large energy storage element to keep the Vcc in the active region for a The output voltage rise time is dependent on the driver output current and the output capacitance. minimum of 50ms (the brown out detect period). The output capacitor is sized for ripple current at maximum load. The NCL30081/83 uses an open Timing Guidelines loop control for output current. Consequently, Poor dimming performance can be traced back to there is no slew rate improvement due to the gain timing race issue. of the feedback loop. The energy in the output 1. Vcc Decay capacitor must be satisfied before the current in 2. Output Voltage rise time the LEDs. Since the output current is fixed as an Vcc Decay When the input power is interrupted, the output decays as the input energy storage is depleted. August 2014, Rev. 0 open loop event, the output voltage rise varies directly as the output current setting. Vcc voltage rise follows the output voltage rise because they www.onsemi.com 2 DN05065/D are coupled through the aux winding of the Design Recommendations for Step transformer. If the output current drive is at the Dimming lowest setting and the output voltage is low, the 1. Operate Vcc as high as practical without Vcc winding may not deliver enough voltage to causing an OVP at the maximum current and bootstrap the Vcc supply before the Vcc capacitor maximum output voltage. gets to Vcc_off. In this case, the circuit gets stuck 2. Use the split Vcc supply to provide energy in an endless loop until the controller is reset to storage for Vcc 100% output. While it may seem as though this is 3. Size the output capacitor to be either small a fundamental limitation, there are some special enough or large enough. characteristics of LEDs that make this scenario 4. Testing with a resistive load will not work work. properly. 1. Very small Output Capacitance 2. Very large Output Capacitance 1 In the first scenario, the output capacitance is chosen to be sufficiently small that the output voltage rises in time to bootstrap the Vcc/aux winding. The LED ripple current will be high at maximum current as a consequence. The second scenario is counter-intuitive because a very large output capacitor would only serve to further slow © 2014 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Frazier Pruett e-mail: [email protected]. the output voltage rise. This would indeed be the case for a resistor load; however, LEDs act more like zener diodes and leave a large residual voltage on the output capacitor during brief power interruptions. It is the residual voltage on the output capacitor that allow the converter to recover at low output currents. Testing with a resistor load is not recommended with a step dimming driver. August 2014, Rev. 0 www.onsemi.com 3