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ISO7721BDW Datasheet(PDF) 21 Page - Texas Instruments
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ISO7721BDW Datasheet(HTML) 21 Page - Texas Instruments
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SLLSEY9A – APRIL 2018 – REVISED APRIL 2019
Product Folder Links: ISO7721B
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Copyright © 2018–2019, Texas Instruments Incorporated
10 Power Supply Recommendations
To help ensure reliable operation at data rates and supply voltages, a 0.1-μF bypass capacitor is recommended
at the input and output supply pins (V
). The capacitors should be placed as close to the supply pins
as possible. If only a single primary-side power supply is available in an application, isolated power can be
generated for the secondary-side with the help of a transformer driver such as Texas Instruments' SN6501 or
SN6505A. For such applications, detailed power supply design and transformer selection recommendations are
available in SN6501 Transformer Driver for Isolated Power Supplies or SN6505 Low-Noise 1-A Transformer
Drivers for Isolated Power Supplies.
11.1 Layout Guidelines
A minimum of four layers is required to accomplish a low EMI PCB design (see Figure 18). Layer stacking should
be in the following order (top-to-bottom): high-speed signal layer, ground plane, power plane and low-frequency
Routing the high-speed traces on the top layer avoids the use of vias (and the introduction of their
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/in
Routing the slower speed control signals on the bottom layer allows for greater flexibility as these signal links
usually 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 it symmetrical. This makes the stack mechanically stable and prevents it from 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.
11.1.1 PCB Material
For digital circuit boards operating at less than 150 Mbps, (or rise and fall times greater than 1 ns), and trace
lengths of up to 10 inches, use standard FR-4 UL94V-0 printed circuit board. This PCB is preferred over cheaper
alternatives because of lower dielectric losses at high frequencies, less moisture absorption, greater strength and
stiffness, and the self-extinguishing flammability-characteristics.
11.2 Layout Example
Figure 18. Layout Example
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