Modern Printed Circuits
High Voltage Drop On the other hand, LEDs pose a host of issues to the PCB layout designer, including thermal management challenges. The most prevalent is the fact that LEDs have a high voltage drop. Depending on the chipmaker, this can run between 3.1 to 4 volts, which is substantially large compared to the normal 1.08 or 2.7 volts. Hence, the PCB designer is looking at a huge voltage drop and they wonder how to incorporate it in his or her design. To maintain the white backlight compared to the earlier green light, battery voltage has to be increased. By increasing battery voltage, the size of the switching regulators is also increased, thus increasing the amount of needed circuitry. There are two ways of doing that; with the integrated LED circuitry, it can be placed close to the LED or separately.
Figure 1: MCPCB
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The common practice today is to separate driver circuitry from the LED assembly to avoid the heat transfer from the LED assembly to the driver circuitry.
Design Considerations There are several key PCB design considerations involved here, but thermal management is foremost among them. Within thermal management, there are a few other key issues to ponder; those involve PCB material, thermal vias versus copper plate, and junction temperature. As far as PCB material, there is the traditional, cost-effective, widely available FR4 versus a metal core (MC) PCB. The biggest issue with FR4 is its poor thermal conductivity. On the other hand, a MCPCB, as shown in Fig. 1, is great thermal conductor, but it costs 20 to 30 percent more than FR4.