AN1212: Advanced Load Driving Considerations for the Si834x Isolated Smart Switch
●The Si834x Isolated Smart Switch devices incorporate a rich set of protection mechanisms and diagnostic reporting capabilities to help ensure long, safe, reliable operation of PLC systems. The Si834x devices offer galvanic isolation to insulate the low voltage PLC controller circuitry, and users, from transients and fault conditions that may bepresent on the PLC output circuits. An integrated ""smart"" multi-voltage demagnetization clamp limits voltage spikes and absorbs demagnetization energy from inductive loads,and protection mechanisms such as temperature sensing, dissipated energy estimation, current monitoring, and under voltage shutdown are included to help PLC systems handle adverse load conditions.
●Introduction:
■The Si834x Isolated Smart switch devices are designed for use in industrial applications such as Programmable Logic Controller (PLC)output circuits. PLC output circuits typically provide switching of 24 VDC supplies to control industrial loads like relays, motor actuators,or lamps. The Si834x isolated switches allow logic signals from low voltage processors to control switching of the higher-voltage PLC outputs while providing galvanic isolation between the logic inputs and the PLC outputs. Galvanic isolation provides protection for the processors and human operators on the low voltage side of the circuit from the higher voltages and possible transients or fault conditions that may arise on the output side of the PLC equipment.
■PLC outputs can be exposed to a wide variety of load conditions that can present many challenges for traditional switch circuits. For example, PLC outputs often drive relay coils or other inductive loads which store energy in their magnetic field while the switch is in the ""on"" state. The interruption of current when the switch turns off can often generate enough ""inductive kick"" or ""flyback"" voltage to damage the switch unless some form of voltage ""clamping"" is implemented. Care must be taken when implementing demagnetization clamp circuits to ensure that absorption of the coil's demagnetization energy does not cause overheating in the clamp. PLC output circuits should also be designed to protect themselves from overheating when the output is accidentally shorted. On the other hand, PLC output may also be used to illuminate light bulbs, which can draw amps of initial “inrush” current before the filament heats up, so these loads may initially look very much like a short circuit.
■The Si834x isolated smart switch devices incorporate a number of innovative protection and diagnostic capabilities which facilitate driving of difficult industrial loads, and which distinguish this family of switches from competing devices. The Si834x 4-channel switches deliver up to 700 mA (IO(ON)) continuously per channel, meeting and exceeding the IEC 61131-2 requirements for 500 mA drive capability. An innovative ""smart"" multi-voltage demagnetization clamp is included on-chip to protect against the over-voltage conditions and absorb the demagnetization energy that can result from driving inductive loads such as relays, solenoids and motors. This integrated demagnetization clamp dynamically switches between operation at 2.1 V (VCLMPL) or 17.5 V (VCLMPH), based on sensed operating current, temperature, and supply voltage. A special ""Inrush"" mode of operation accommodates up to 8 A (IOPCL) of peak inrush current for loads such as lamps. Inrush mode is complemented by a ""Safe Start"" mode of operation which uses an integrated power estimation technique to prevent excessive power dissipation or overheating when exposed to a short circuited output or other elevated current conditions for extended time periods. The Si834x on-chip diagnostics also detect and report open-circuit output conditions back to the host.
■The low-voltage logic interface side of the Si834x operates at a VDD1 supply range of approximately 2.25 to 5.5 V, and the switch side of the Si834x operates over a VDD2 supply range of approximately 9 to 32V. Undervoltage protection is provided for both the VDD1 and the VDD2 supplies. In addition, an overvoltage condition on the VDD2 supply will constrain operation of the clamp to help avoid excessive power dissipation.
■PLC systems may require sourcing or sinking output switch configurations, depending on the application. The Si834x devices are available in both sourcing and sinking switch configurations, as depicted in the figure below. The sourcing configuration operates as an open-source output for high-side switching. The sinking configuration uses an open-drain output for low-side switching. Both the sourcing and sinking configurations of the Si834x isolated smart switch provide galvanic isolation between the low-voltage logic interface and the switch outputs.
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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July 26, 2021 |
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Rev. 0.1 |
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AN1212 |
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860 KB |
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