 |
 |
|
| DN86 |
|
||||||||||||||||
 ZETEX
|
| ||||||||||||||||
|
4 page 
DN86 © Zetex Semiconductors plc 2007 Circuit board views Top Layer Bottom Layer Figure 4 Circuit board views Choice of Inductor and switching circuit layout A 100 µH screened inductor was chosen to set the nominal frequency around 250kHz. A screened inductor is chosen to minimize radiated EMI. The layout with any switching regulator is crucial to minimize radiated EMI. This reference design keeps the critical track lengths to a minimum. Ground areas have been maximized around critical areas. Circuit performances Circuit performances have been evaluated taking into account two main parameters, the system efficiency and the current precision. The reference current is set to a nominal 300mA but can be adjusted to any value up to 350mA by changing the sense resistor Rsense according to the formula: Iref = 0.1/R1 [A] For R1 = 0.33 Iref = 300mA In Table 2 the data related to the system supplied with a DC voltage ranges from 12V to 15V. For these tests the Schottky bridge was included. The most important parameters are the system efficiency and the error between the rated LED current (300mA) and the actual LED current. In the DC case the frequency ranges between 150kHz and 300kHz, depending on the input voltage. Whatever the input voltage, the efficiency is higher than 87% and the error lower than 2%. Table 3 shows the data related to the system supplied with an AC electromagnetic transformer. Using a SMD tantalum capacitor will save space and avoid using a larger aluminum electrolytic capacitor. This will improve the reliability of the system and stabilize performance during its lifetime. There is a trade off between physical size, reliability, cost and average LED current. Typical output voltages from a nominal 12V AC transformer can be ±10%. With 3 LEDs the voltage across these will be around 10V. If the input capacitor value is lower then 200µF, the AC input waveform is distorted (as can be seen in figure 8). When the rectified AC is not sufficiently Vin [V] Iin[A] Vout[V] Iout[A] Efficiency Current Accuracy 12.000 0.275 9.80 0.296 87,9% 1.3% 13.000 0.252 9.78 0.294 87.7% 2.0% 14.000 0.232 9.76 0.294 87.6% 2.0% 15.000 0.220 9.75 0.294 87.4% 2.0% Table 2 DC input voltage MR16 Pins - Anode LED connections + Cathode |
