UCD3138RGCT datasheet(76/99 Pages) TI2

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12.1.5 UART Communication Port

UART is used for communicating between the primary side and secondary side with isolation boundary.

Normally, the communication wires are long. These wires can easily be interfered by EMI and pick up noise

of switching power supplies. First, the wires must be routed without directly exposing the traces to the switching

noise source, and then a termination is needed at the end of the trace, as shown in Figure 12-6. For example, R

= 50 Ω, C = 47 pF if they don’t significantly slow down the slew rate of the signals. When the pins are not used,

tie them to the single ground plane SGND.

Figure 12-6. Termination for Communication Port (UART)

12.1.6 Special Considerations

• The first thing that must be done in any layout is to set up the basic grounding strategy and the placement

of the decoupling capacitors. This needs to be prioritized over anything else, even the routing of sensitive

feedback signals.

• If a gate driver device such as UCC27524 or UCC27511 is on the control card and there is a PGND

connection, a net-short resistor or large copper trace must be used to tie the PGND to the Power RTN by

multiple vias. Also, the net-short element between Power RTN and PGND must be close to the driver IC.

• Unused ADC pins must be tied to SGND.

• Avoid putting V33D and V33A long traces or planes close to radiation components. Place them into an

internal layer. It is preferred to have ground shielding.

• Avoid putting bias supplies or SGND or Power RTN directly to across the switching power train where they

can couple switching noise. If the grounds are coupled with noise, the decoupling capacitors may not be

effective at filtering the noise out.

• Local capacitors are preferred to provide a short path for switching current, and be careful to select a quiet

RETURN point to connect.

• In a power module or a tiny PCB design, a single solid plane without the grounding separation is shown in

Figure 12-7and has a single point connection with power RTN or SGND near the connector. Ensure there is

no current flow from power train into the signal ground plane.

UCD3138

SLUSAP2J – MARCH 2012 – REVISED NOVEMBER 2021

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