AN2536 Freescale Semiconductor / Motorola, AN2536 Datasheet - Page 5

• Manufacturer Part Number: AN2536
• Description: MC9328MX1 and MC9328MXL High Speed Layout Design Guidelines
• Manufacturer: Freescale Semiconductor / Motorola

In Figure 5 through Figure 7, we found the impedance is inversely proportional to trace width and directly
proportional to trace height above the ground plane. However, the rate of change with trace height above
GND is much slower in a stripline layout compared with a microstrip layout. A stripline layout has a signal
sandwiched by FR-4 material, whereas a microstrip layout has one conductor open to air. This exposure
causes a higher, effective dielectric constant stripline layout compared to microstrip layouts. Thus, to
achieve the same impedance, the dielectric span must be greater in stripline layouts compared with
microstrip layouts. Therefore, stripline layout PCBs with controlled impedance lines are thicker than
microstrip layout PCB.
Equation 7 and Equation 8 are used to calculate the relationship between propagation delay (tPD) or the
time required for a signal to travel from one point to another. Transmission line propagation delay is a
function of the dielectric constant of the material.
Microstrip Layout Propagation Delay:
Stripline Layout Propagation Delay:
MOTOROLA
Figure 7. Stripline Trace Impedance vs. Trace Thickness (T)
Figure 6. Stripline Trace Impedance vs. Trace Height (H)
Freescale Semiconductor, Inc.
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MC9328MX1/MXL Application Note
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This figure shows the stripline
trace impedance vs. trace
height (H), using the values in
Equation 5, keeping trace
width and trace thickness
constant.
This figure shows the
stripline trace impedance
vs. trace thickness (T)
using the values in
Equation 5, keeping trace
width and dielectric
height constant.
Design Consideration
Eqn. 7
Eqn. 8
5