> 5 | Layout Guidelines

> 5.1 | General Guidelines

> The PCB layout is very critical in a power supply design, and should be considered as one of the most important design issues. A PCB strip line has to be viewed as a resistance as well as an inductance. The resistance for a strip line can be calculated with the formula:
> Resistivity x Length / Area
> Resistivity The resistance for copper, which is 0,017μOhm x m @ 20 degrees
> Length The length of the lead
> Area Width x copper thickness
> Examples:
> In this first example the strip line on the PCB is 0,5mm wide, 100mm long and has a 0,035mm copper thickness. The resistance can be calculated as:
> R = 0,017 x 0,1m / (0,5 x 0,035)mm 2
> R = 0,0017 / 0,0175 = 0,097 Ohm
> The resistance in this lead is almost 100mOhm; more than enough to have an influence on the power supply line.
> In the second example the strip line on the PCB is 2mm wide, and 20mm long. The resistance can be is calculated as:
> R = 0,017 x 0,02 m / (2 x 0,035) mm 2
> R = 0,00034 / 0,07 = 0,005 Ohm
> The resistance is 5mOhm, which is almost neglectable.
> The conclusion from these examples is obvious. The lead from the power supply IC and/or capacitor shall be as short and wide as possible.
> NOTE: In the examples above it is assumed that the copper thickness on the PCB is 35μm. Normal values of copper thickness can be from 15μm to 50μm. If the copper thickness had been 15μm instead of 35μm in the examples above, the resistance had been 2,3 times higher.
> The figure below shows a power supply line on the PCB. It is a wide line, but with short narrow lines to each soldering pad to a module’s connector. A wide line across all power supply lines will decrease the solderability.
> Figure 6: Power feeding line
> As previous mentioned a strip line could also be seen as an inductor. In the figure below the power supply-feeding line is placed in parallel with a microphone line. If those lines are seen as inductors they can be viewed as a transformer. The efficiency of the transformer depends on the frequency, distance and length of the line. So, if the small line were a data line, it might not have any impact. But since it is a microphone line, some problems can arise from this kind of design.
> Figure 7: Inductance of power feeding line