AN1212: Advanced Load Driving

Considerations for the Si834x Isolated

Smart Switch

Output circuits in industrial programmable logic controllers

(PLCs) can be routinely subjected to adverse load conditions.

PLC outputs often drive relay coils or other inductive loads that

can generate potentially dangerous voltage spikes and must dis-

charge significant energy from their magnetic fields when switch-

ed off. Loads such as lamp filaments may require amps of inrush

current, and short-circuited outputs must also be tolerated. These

adverse load conditions can damage the PLC output switch de-

vice, and damage may spread to the host controller interface, po-

tentially putting the PLC controller and possibly system operators

at risk, if adequate protection mechanisms are not in place.

The Si834x Isolated Smart Switch devices incorporate a rich set of protection mecha-

nisms 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 be

present 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 estima-

tion, current monitoring, and undervoltage shutdown are included to help PLC systems

handle adverse load conditions.

KEY POINTS

• Galvanic isolation insulates low-voltage

host interface from industrial loads

• Integrated “smart” multi-voltage

demagnetization clamp

• Short-circuit and over-current protection

• Undervoltage fault detection

• Open-circuit detection

• Diagnostic reporting

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1. 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 condi-

tions 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 dam-

age 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 out-

put 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 driv-

ing of difficult industrial loads, and which distinguish this family of switches from competing devices. The Si834x 4-channel switches

deliver up to 700 mA (I

O(ON)

) continuously per channel, meeting and exceeding the IEC 61131-2 requirements for 500 mA drive capa-

bility. 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 (V

CLMPL

) or 17.5 V (V

CLMPH

), based on sensed operating cur-

rent, temperature, and supply voltage. A special “Inrush” mode of operation accommodates up to 8 A (I

OPCL

) 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 avail-

able 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 sourc-

ing and sinking configurations of the Si834x isolated smart switch provide galvanic isolation between the low-voltage logic interface and

the switch outputs.

VDD2

GND2

Load

Smart

Output

Clamp

Control

Switch

Control

Figure 1.1. Si834x Sourcing Device

VDD2

GND2

Load

Smart

Output

Clamp

Control

Switch

Control

Figure 1.2. Si834x Sinking Device

The balance of this application note provides more detailed information about the advanced switch protection and diagnostic features of

the Si834x family of isolated smart switches. Section 2 provides an overview of the various sensing and protection mechanisms imple-

mented in the Si834x isolated smart switch devices. Section 3 describes the integrated “smart” multi-voltage demagnetization clamp,

and section 4 presents a brief description of the state machine that is involved in the implementation of these protection mechanisms.

The descriptions in the following sections generally apply to the default behavior of one channel of a 4-channel sourcing configuration

AN1212: Advanced Load Driving Considerations for the Si834x Isolated Smart Switch

Introduction

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