9.2.2 Detailed Design Procedure

Power isolation is accomplished using the push-pull transformer driver SN6501, a low-cost LDO and TLV70733.

Signal isolation uses the quadruple digital isolator ISO7741. Notice that both enable inputs, EN1 and EN2, are pulled-up via 4.7-kΩ resistors to limit input currents during transient events.

While the transient protection is similar to the one in Figure 28 (left circuit), an additional high-voltage capacitor diverts transient energy from the floating RS-485 common further towards protective earth (PE) ground. This diversion is necessary as noise transients on the bus are usually referred to Earth potential.

R_VH refers to a high-voltage resistor, and in some applications even a varistor. This resistance is applied to prevent charging of the floating ground to dangerous potentials during normal operation.

Occasionally varistors are used instead of resistors in order to rapidly discharge C_HV, if expected that fast transients might charge C_HV to high-potentials.

Note that the PE island represents a copper island on the PCB for the provision of a short, thick Earth wire connecting this island to PE ground at the entrance of the power supply unit (PSU).

In equipment designs using a chassis, the PE connection is usually provided through the chassis itself. Typically the PE conductor is tied to the chassis at one end while the high-voltage components, C_HV and R_HV, are connecting to the chassis at the other end.