ISO7740FDWR datasheet(35/52 Pages) TI2

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8.4 Layout

8.4.1 Layout Guidelines

A minimum of four layers is required to accomplish a low EMI PCB design (see Figure 8-8). Layer stacking must

be in the following order (top-to-bottom): high-speed signal layer, ground plane, power plane and low-frequency

signal layer.

• Routing the high-speed traces on the top layer avoids the use of vias (and the introduction of the

inductances) and allows for clean interconnects between the isolator and the transmitter and receiver circuits

of the data link.

• Placing a solid ground plane next to the high-speed signal layer establishes controlled impedance for

transmission line interconnects and provides an excellent low-inductance path for the return current flow.

• Placing the power plane next to the ground plane creates additional high-frequency bypass capacitance of

approximately 100 pF/inch2.

• Routing the slower speed control signals on the bottom layer allows for greater flexibility as these signal links

typical have margin to tolerate discontinuities such as vias.

If an additional supply voltage plane or signal layer is needed, add a second power or ground plane system

to the stack to keep the planes symmetrical. This makes the stack mechanically stable and prevents warping.

Also the power and ground plane of each power system can be placed closer together, thus increasing the

high-frequency bypass capacitance significantly.

For detailed layout recommendations, refer to the Digital Isolator Design Guide.

8.4.1.1 PCB Material

For digital circuit boards operating below 150 Mbps, (or rise and fall times higher than 1 ns), and trace lengths

of up to 10 inches, use standard FR-4 UL94V-0 printed circuit boards. This PCB is preferred over cheaper

alternatives due to the lower dielectric losses at high frequencies, less moisture absorption, greater strength and

stiffness, and self-extinguishing flammability-characteristics.

8.4.2 Layout Example

10 mils

40 mils

FR-4

0r ~ 4.5

Keep this

space free

from planes,

traces, pads,

and vias

Ground plane

Power plane

Low-speed traces

High-speed traces

Figure 8-8. Layout Example Schematic

www.ti.com

ISO7740, ISO7741, ISO7742

SLLSEP4J – MARCH 2016 – REVISED OCTOBER 2024

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