One-Wire Serial Bus Carries Isolated Power And Data

Posted on Feb 5, 2014

Medical and industrial applications often require galvanic isolation of 2500 V ac or higher for the safety of patients and equipment operators. The isolation barrier conveys not only power to the sensing element, but also data to or from that element. Each data signal crossing the barrier-requires isolation. Consequently, designers can typically s

One-Wire Serial Bus Carries Isolated Power And Data
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ave costs in these applications by choosing a serial bus rather than a parallel bus. Serial buses include SPI, I2C, and the Dallas 1-Wire bus. The Dallas 1-Wire requires only one data line (plus ground) for bidirectional-communications. Because optoisolators are unidirectional devices, typical 1-Wire applications require two optos ”one for each direction of data flow. (SPI and I2C applications require a minimum of three optos. ) The 1-Wire bus not only allows bidirectional data flow, it also can transfer power in its "parasitic power mode. " An isolated converter supplies power for the sensing element. Thus, most 1-Wire designs require two optos for the data interface: one opto for feedback to the isolated power supply, and a transformer for power-supply isolation. The circuit shown in the figure, implemented with an isolated transformer driver (U1) and a single optocoupler (U3), minimizes the component count required for isolation while maintaining a two-wire pigtail design (data plus ground to the 1-Wire sensing element). U1 provides isolated and pseudoregulated power, and it enables the master to transmit data to the 1-Wire device across an isolated interface. The single opto`s location in the receive datapath lets the master receive isolated data from the 1-Wire device. Note the following observations: U1 and transformer T1 generate approximately 4. 0 V at the VISOL node (upper right corner of the schematic). D1 and D2...

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