RT8487 is a high power factor LED driver controller that can be used in non-isolated Buck and Buck-Boost applications. The controller uses resonant switching in boundary conduction mode. The below application example shows a 10W design using sixteen Cree XLAMP MX-6 LED in series for 49V LED string voltage.
Figure 11. RT8487 Non-isolated 10W high PF LED driver
The driver is designed for 200mA average output current. The % flicker is set at 30%. The LED ripple current and voltage can be derived from below graphs.
Figure 12. LED characteristics for Cree XLAMP MX-6
From the luminance vs. current graph: For 30% light flicker, the LED current may vary +/- 70mA or 35%.
From the I/V curve: The dynamic resistance of one LED around the 200mA operation point is 1.7Ω. The dynamic resistance of the total LED string assembly is 1.7*16 = 27.2Ω (note that the dynamic resistance of the LED is higher in the low current range). The allowed voltage ripple across the total string combination can be calculated:
Figure 13 shows the LED driver schematic. RT8487 is used in Buck topology with floating controller. A bootstrap circuit provides the IC supply, and the circuit only uses standard drum coil inductors.
Figure 13. RT8487 10W high PF design in floating Buck topology
For a 50Hz line frequency design, the required output capacitor value can be estimated by:
A 220uF output capacitor was selected for EC1.
Figure 14 shows the output measurements for this converter.
Figure 14.
The purple waveform is the Buck converter output current with all high frequency switching removed. The AC amplitude at 424mApp is slightly higher than double the average LED current. The LED voltage ripple is 3.07Vpp, a bit lower than the originally calculated value due to the larger output capacitor. The average LED current is 200mA with ripple of 120mApp, which is 30% ripple. The dynamic resistance of the LED assembly is slightly lower than calculated from the LED graphs: 3.07V/120mA = 25.6Ω.
The scope plot on the right side shows LED current and measured light output (light measurement done with the self-made light sensor, see chapter 9). The measured flicker is 26.1%, well below the max requirement.
Figure 11. RT8487 Non-isolated 10W high PF LED driver
The driver is designed for 200mA average output current. The % flicker is set at 30%. The LED ripple current and voltage can be derived from below graphs.
Figure 12. LED characteristics for Cree XLAMP MX-6
From the luminance vs. current graph: For 30% light flicker, the LED current may vary +/- 70mA or 35%.
From the I/V curve: The dynamic resistance of one LED around the 200mA operation point is 1.7Ω. The dynamic resistance of the total LED string assembly is 1.7*16 = 27.2Ω (note that the dynamic resistance of the LED is higher in the low current range). The allowed voltage ripple across the total string combination can be calculated:
Figure 13 shows the LED driver schematic. RT8487 is used in Buck topology with floating controller. A bootstrap circuit provides the IC supply, and the circuit only uses standard drum coil inductors.
Figure 13. RT8487 10W high PF design in floating Buck topology
For a 50Hz line frequency design, the required output capacitor value can be estimated by:
A 220uF output capacitor was selected for EC1.
Figure 14 shows the output measurements for this converter.
Figure 14.
The purple waveform is the Buck converter output current with all high frequency switching removed. The AC amplitude at 424mApp is slightly higher than double the average LED current. The LED voltage ripple is 3.07Vpp, a bit lower than the originally calculated value due to the larger output capacitor. The average LED current is 200mA with ripple of 120mApp, which is 30% ripple. The dynamic resistance of the LED assembly is slightly lower than calculated from the LED graphs: 3.07V/120mA = 25.6Ω.
The scope plot on the right side shows LED current and measured light output (light measurement done with the self-made light sensor, see chapter 9). The measured flicker is 26.1%, well below the max requirement.
