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PCBD-May2015

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12 The PCB Design Magazine • May 2015 CONTROLLED IMPEDANCE DESIGN continues For a microstrip transmission line with 3 mil dielectric thickness to ground, a 9 mil trace width is required to match a DDR3 34 ohm driver. Have you ever tried routing matched length DDR3 with 9 mil traces? That ain't gonna work. If the driver's impedance is even lower, say, 22 ohms, then you would need to route 15 mil traces. Figure 2 illustrates the plot of impedance vs microstrip trace width (left) and impedance vs dielectric thickness (right). These plots are sim- ulated by multiple passes of the field solver (in the background) to create heads-up graphs of how to adjust the particular variables to achieve the desired impedance. One can see that as the impedance goes down, the trace width increas- es to a point where it will be unroutable. Also, if we select too low of an impedance, the di/ dt will increase, drawing excessive current from the supply and no doubt creating further power integrity issues. So, it is a trade-off between trace width, trace (copper) thickness, dielectric thickness Figure 1: unterminated (red) vs. terminated (blue) transmission lines. beyond design

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