LM3409HV MR16 Buck-Boost Reference Design for Non-Dimming & Dimming LED Applications (LM3409HV-REF)
This new LED MR16 will replace the standard halogen by driving 3 high power LEDs. This following report describes the design and shows practical test results of this LED-based MR16 designed to drive 3 LEDs in series from an AC input voltage of 11.5VAC rms generated by 50Hz magnetic transformers or Electronic transformers. Halogen MR16 bulbs do not flicker because the light emitted depends on the heat of the wire in the bulb. The heat changes more slowly compared with the change in current (50Hz) from the mains. The light emitted by LEDs changes instantly with the current. Therefore special measures need to be taken to avoid flickering. The solution is to create a system that has a power factor close to 1 to get the same behavior as the halogen bulb. Several existing approaches use a large value electrolytic capacitor after the bridge rectifier to smooth the AC waveform. This has also the advantage of keeping an output voltage higher than the forward voltage of the LEDs and provides a continuous supply voltage (Vcc) for the driver. This approach works really well with 50 Hz transformers and provides power factors of up to 0,65 .Using this method with an electronic transformer will make the LED flicker. To keep a good power factor we need to get rid of the big electrolytic capacitor after the bridge rectifier. By eliminating this capacitor, a half-sine waveform will be available going from 0V up to 12Vв€љ2. Consequently it will switch the driver on and off unless we can provide a circuit that works down to a 0V input. One of the solutions is to use a power factor correction stage and then an LED driver, but due to the board size and cost this is not the right approach. The best solution is to use a buck boost topology with the LM3409 LED driver with constant off time. With this solution, a 0.98 PF is achievable by using the IADJ pin of the IC connected via a resistor divider to the AC waveform and creating a sine-wave input current that is proportional to the input voltage. This buck boost topology provides a constant negative forward voltage for the LEDs. This forward LED voltage is applied to the IC in addition to the input voltage that supplies the driver. Therefore when the input voltage Vac reaches 0V, the minimum voltage available to the IC is three times the forward voltage of an LED. When the input voltage reaches its max peak value of 12Vв€љ2, the maximum voltage available to the driver IC is three times the forward voltage of an LED in addition to 12Vв€љ2. With this approach, we keep the driver always ON even if the input voltage reaches 0V. Phase dimming may be an option with certain combination of electronic transformer and Triac dimmer.
Tested with Electronic Transformers (12VAC output) Work with off the shelf dimmers with no flicker, stable operation Flicker-free operation with common Electronic Low Voltage dimmers, and Electronic Transformers