Robust and ultra-low power isolation for digital communication interfaces

As the complexity of factory automation networks increases, multiple components and modules are usually connected for control and communication. Such discrete systems will face various surge shocks or noise interference common in industrial environments. Therefore, designers need to provide a signal isolation solution that can not only prevent transient high voltage insulation protection but also prevent common mode noise interference. Although traditional optoelectronic isolation technology can already meet this demand, new design trends have put forward the requirement of lower power consumption.

Digital communication interface isolation

Electrical isolation is to prevent charged particles from moving from one circuit to another. There should be no physical conductive connection between the power supply, grounding and circuits between two isolated circuits. At the same time, the necessary information between the two circuits can be transmitted and exchanged by other means, such as using optoelectronic conversion.

There are four main reasons for the need for isolation in digital communication: First, it can be used to isolate and protect the instantaneous high voltage surge generated by one module from damaging another module. This is particularly important in industrial environments because large high-voltage mechanical equipment is usually connected to each other through communication buses. Second, isolators can also be used to convert signal levels or adjust the level logic signals between systems or circuits working with different power supply voltages. For example, a microprocessor may operate with a 3.3V power supply, while an I/O device may operate with a 5V power supply. Optocouplers can be used to connect these different modules. Third, isolators can also be used to suppress common-mode transient noise. Common-mode noise may cause abnormal voltage changes or excessive noise in the output signal. The coupling capacitance of the optocoupler is relatively small, which can directly and effectively reduce the common-mode voltage and ensure the quality of the signal. The suppression of common-mode noise is very important for circuits that are susceptible to interference, such as sensors, A/D converters, signal transmitters, etc. Finally, isolators can also be used to cut off ground loops caused by different ground levels. Ground loops cause unnecessary current flow between two points on a common path in an electrical system. If the ground loop current is not eliminated, it will seriously affect the integrity of the signal and even trigger an erroneous signal.

Figure 1. Typical isolated SPI interface

Low-power isolation solution for digital communication

Optocouplers are widely used in the industrial field. They can be used to isolate fieldbus systems such as Profibus, DeviceNet, USB, I2C, RS485 and CANBus. Optocouplers can provide high-voltage surge protection for the entire system, help eliminate noise in ground loops, and can also be used to isolate feedback signals or PWM drive signals between the primary and secondary sides of switching power supplies. In addition, optocouplers can also be used to provide isolation and noise control for the interface between the hot-swap controller and the main controller in POE (Power-Over-Ethernet), that is, Ethernet power supply.

In factory automation, the control unit must communicate with the outside world. Since there are many interconnected devices at different network layers and physical locations, ground loops and high transient short pulses are very common in fieldbus applications. High-speed digital optocouplers with high common-mode rejection are required to isolate and protect these systems. Proper isolation ensures that the digital signals provided by the system MCU or DSP can safely and accurately control the motor.

The SPI serial peripheral interface is a widely used communication bus that allows multiple slaves to connect to a master for communication and control. SPI uses three main logic signals, namely serial clock, signal input (SDI) and signal output (SDO). If only a single slave is used, the fourth signal line, slave select (SS), can be set to low. SPI can be used to replace parallel interfaces, avoiding the trouble of parallel circuits and wrapping PCBs. For example, a 12-bit ADC with SPI output can reduce 12 signal lines to a single SPI output, but the price paid is the effective data transmission speed. Assuming the same serial speed is used, the effective speed will be reduced by 12 times. Figure 1 provides a typical isolated SPI interface. To transfer data, the master must first use a clock configuration that is lower than or equal to the highest frequency that the slave device can support. In each SPI clock cycle, full-duplex data transmission occurs. The master sends a bit of information on the SDI line, and the slave complements it on the same line. The slave sends a bit of data on the SDO line, and the master also complements it with the corresponding line. In the entire SPI data transmission, loop delay is very important for the normal operation of the system. The low-power, high-speed optocoupler series uses fast CMOS technology and has a total loop delay of approximately 92ns.


When the optocoupler is working, there are two main components that consume current, namely the forward current IF that drives the LED and the detector current ID that converts the LED's optical signal into an electrical signal. The optocoupler can achieve low power consumption by reducing the LED drive current and detector current. Power efficiency is often a key performance parameter that engineers must continuously strive to improve. There are four important reasons to choose a low-power optocoupler: first, it can reduce overall power consumption; second, lower power consumption can reduce heat, thereby simplifying the design of temperature management systems; third, by using a lower drive current, the LED life of the optocoupler can be greatly extended; finally, the LED drive current below 4mA can be directly driven by most microcontrollers and ASICs without the need for an external buffer.

Figure 2 shows the field service life and current conversion ratio (CTR) degradation of Avago's optocoupler when operating at a drive current of 2mA at 85℃ to 105℃. Basically, the working life of the LED is inversely proportional to the drive current. By reducing the drive current, the working life of the LED can be greatly improved. The figures in Figure 2 use the equivalent field hours calculated using Black's Model, based on 100% LED illumination, using three sigma data, and a 5% degradation in Avago's LED CTR, which allows for up to 35 years of continuous operating life.

Figure 2 Field service life and current conversion ratio (CTR) degradation of Avago optocouplers at 85°C to 105°C and 2mA operating current

Innovative Design Techniques and Features

There are several important benefits to using optocouplers. In addition to providing a direct connection to an MCU or DSP, it is also easy to change the polarity of the output. Most importantly, optocouplers provide excellent noise immunity, making them ideal for use in high-noise electrical environments.

Previous optocouplers may require a buffer to be connected to the output of the MCU or DSP to increase the drive current of the LED. Currently, Avago's latest optocouplers can be lit with a forward current as low as 40μA. This type of optocoupler can be driven directly by most microprocessors without the need for an external buffer, thereby saving the number of components and simplifying the board design.

Most isolators have pre-set output configurations, such as inverting or non-inverting. Inverting means that the output signal is opposite to the input signal polarity. For example, when the input logic is high, the output logic is low. Non-inverting means that the output signal has the same polarity as the input signal. LED optocouplers can change the output polarity without using an inverter. In Figure 3, the circuit connection method on the left can provide an inverted output. By connecting Vin to Vcc and grounding to Vin, it can be As the complexity of factory automation networks increases, multiple components and modules are usually required to be connected for control and communication. This discrete system will face various surge shocks or noise interference common in industrial environments. Therefore, designers need to provide a signal isolation solution that can not only prevent transient high voltage insulation protection but also prevent common mode noise interference. Although traditional optoelectronic isolation technology can already meet this demand, new design trends have put forward requirements for lower power consumption. Digital

communication interface isolation

Electrical isolation is to prevent charged particles from moving from one circuit to another. Between two isolated circuits, there must be no physical conduction connection between their power supply, grounding, and circuits. At the same time, the necessary information between the two circuits can be transmitted and exchanged in other ways, such as using optoelectronic conversion.

There are four main reasons for the need for isolation in digital communication: First, it can be used to isolate and protect the instantaneous high voltage surge generated by one module from causing damage to another module. This is particularly important in industrial environments because large high-voltage mechanical equipment is usually connected to each other through a communication bus. Second, isolators can also be used to convert signal levels or adjust the level logic signals between systems or circuits working with different power supply voltages. For example, a microprocessor may work with a 3.3V power supply, while an I/O device may work with a 5V power supply. Optocouplers can be used to connect these different modules. Third, isolators can also be used to suppress common-mode transient noise. Common-mode noise may cause abnormal voltage changes or excessive noise in the output signal. The coupling capacitance of the optocoupler is relatively small, which can directly and effectively reduce the common-mode voltage and ensure the quality of the signal. The suppression of common-mode noise is very important for circuits that are susceptible to interference, such as sensors, A/D converters, signal transmitters, etc. Finally, isolators can also be used to cut off ground loops caused by different ground levels. Ground loops cause unnecessary current flow between two points on a common path in an electrical system. If the ground loop current is not eliminated, it will seriously affect the integrity of the signal and even trigger an erroneous signal.

Figure 1. Typical isolated SPI interface

Low-power isolation solution for digital communication

Optocouplers are widely used in the industrial field. They can be used to isolate fieldbus systems such as Profibus, DeviceNet, USB, I2C, RS485 and CANBus. Optocouplers can provide high-voltage surge protection for the entire system, help eliminate noise in ground loops, and can also be used to isolate feedback signals or PWM drive signals between the primary and secondary sides of switching power supplies. In addition, optocouplers can also be used to provide isolation and noise control for the interface between the hot-swap controller and the main controller in POE (Power-Over-Ethernet), that is, Ethernet power supply.

In factory automation, the control unit must communicate with the outside world. Since there are many interconnected devices at different network layers and physical locations, ground loops and high transient short pulses are very common in fieldbus applications. High-speed digital optocouplers with high common-mode rejection are required to isolate and protect these systems. Proper isolation ensures that the digital signals provided by the system MCU or DSP can safely and accurately control the motor.

The SPI serial peripheral interface is a widely used communication bus that allows multiple slaves to connect to a master for communication and control. SPI uses three main logic signals, namely serial clock, signal input (SDI) and signal output (SDO). If only a single slave is used, the fourth signal line, slave select (SS), can be set to low. SPI can be used to replace parallel interfaces, avoiding the trouble of parallel circuits and wrapping PCBs. For example, a 12-bit ADC with SPI output can reduce 12 signal lines to a single SPI output, but the price paid is the effective data transmission speed. Assuming the same serial speed is used, the effective speed will be reduced by 12 times. Figure 1 provides a typical isolated SPI interface. To transfer data, the master must first use a clock configuration that is lower than or equal to the highest frequency that the slave device can support. In each SPI clock cycle, full-duplex data transmission occurs. The master sends a bit of information on the SDI line, and the slave complements it on the same line. The slave sends a bit of data on the SDO line, and the master also complements it with the corresponding line. In the entire SPI data transmission, loop delay is very important for the normal operation of the system. The low power, high speed optocoupler series uses fast CMOS technology with a total loop delay of approximately 92ns.

When the optocoupler is working, there are two main components that consume current, namely the forward current IF that drives the LED and the detector current ID that converts the LED's optical signal into an electrical signal. Optocouplers can achieve low power consumption by reducing the LED drive current and detector current. Power efficiency is often a key performance parameter that engineers must continuously strive to improve. There are four important reasons to choose a low-power optocoupler: first, it can reduce overall power consumption; second, lower power consumption can reduce heat, thereby simplifying the design of temperature management systems; third, by using a lower drive current, the LED life of the optocoupler can be greatly extended; finally, the LED drive current below 4mA can be directly driven by most microcontrollers and ASICs without the need for an external buffer.

Figure 2 shows the field service life and current conversion ratio (CTR) degradation of Avago's optocoupler when operating at a drive current of 2mA under conditions of 85℃ to 105℃. Basically, the operating life of the LED is inversely proportional to the drive current. By reducing the drive current, the operating life of the LED can be greatly improved. The figures in Figure 2 use the equivalent field hours calculated using Black's Model, based on 100% LED illumination, using three sigma data, and a 5% degradation in Avago's LED CTR, which allows for up to 35 years of continuous operating life.

Figure 2 Field service life and current conversion ratio (CTR) degradation of Avago optocouplers at 85°C to 105°C and 2mA operating current

Innovative Design Technology and Functionality

There are several important benefits to using optocouplers. In addition to providing direct connection to an MCU or DSP, it is also easy to change the polarity of the output. Most importantly, optocouplers provide excellent noise immunity, making them ideal for use in high-noise electrical environments.

Previous optocouplers may require a buffer to be connected to the output of the MCU or DSP to increase the drive current of the LED. Currently, Avago's latest optocouplers can be lit with a forward current as low as 40μA. This type of optocoupler can be driven directly by most microprocessors without the need for an external buffer, thereby saving the number of components and simplifying the board design.

Most isolators have pre-set output configurations, such as inverting or non-inverting. Inverting means that the output signal is opposite to the input signal polarity. For example, when the input logic is high, the output logic is low. Non-inverting means that the output signal has the same polarity as the input signal. LED optocouplers can change the output polarity without using an inverter. In Figure 3, the circuit connection on the left can provide an inverting output. By connecting Vin to Vcc and ground to Vin, a non-inverting output can be obtained, which also helps to reduce the number of components.

Figure 3 Using an LED input optocoupler to change output polarity

Common-mode noise is a significant problem in digital communications applications, especially in environments where motors, sensors, and programmable controllers (PLCs) are interconnected. Isolators help reduce noise and enhance signal integrity in such systems. All isolators, regardless of the technology used, generate parasitic capacitance between the two isolated terminals of the device. Noise changes occurring at the output can cause unwanted voltage rises at the input, resulting in false triggering of the input or even input lockup of low-impedance logic. Optocouplers with LED inputs are well suited for applications with high common-mode noise. First, the attenuation of optical signals through light-transmitting insulating materials is very low, so the isolation distance can be increased. The direct benefit of a larger isolation distance is that relatively low parasitic capacitance can be achieved. In other words, unwanted coupling between the input and output terminals can be minimized. Second, by using a split resistor input drive method, the original single current-limiting resistor is divided into two small resistance resistors connected at both ends of the LED, which can balance the impedance on the LED input. In this configuration, the voltage rise on the LED input caused by common-mode noise will be relatively equal, so the LED will not be lit. Third, the LED input has a relatively high input capacitance, typically 70pF. The LED and current limiting resistor in series can act as a low-pass filter to filter out high-frequency noise. By using a balanced shunt resistor, the common-mode rejection ratio of the optocoupler can be increased to 35kV/ms even when the drive current is only 2mA. Common

-mode rejection on isolators has two forms: static and dynamic. Static CMR refers to the ability to reject common-mode noise when the input is fixed high or low, usually when the system is in standby or standby mode. In these states, some components of the system are turned off to save power, leaving only some modules to detect signals. The system must maintain the same logic state regardless of whether static common-mode noise occurs in the environment. This is mainly to ensure that noise does not cause false triggering of the system. In a dynamic environment, the system continuously sends signals through logical high and low levels. To prevent common-mode noise from coupling to the input signal, the system must filter out this noise, which is called dynamic CMR. For some isolators, the dynamic CMR of the system is usually lower than the static CMR. Avago's optocoupler detects the forward current set by the input signal and outputs light to the detector, so its structure determines that the performance in dynamic and static environments is equivalent.

Avago's optocouplers can meet the requirements of high-speed devices in terms of numbers. The optocouplers for LED inputs have the ability to connect a peaking capacitor in parallel to the input current limiting resistor to improve speed performance. The size of the peaking capacitor can be determined by the rise and fall time of the input signal, the power supply voltage, and the input current drive of the LED. In Figure 4, the left figure is a circuit of a power supply capacitor, and the right figure is the test result. It can be seen from the figure that transmission delay and pulse width distortion can be improved by adding a peaking capacitor. Among them, the solid line part is the performance without adding a peaking capacitor, and the dotted line part is the improvement after adding a peaking capacitor.

Figure 4: By adding a peaking capacitor in parallel with the series input current limiting resistor, the speed performance (tplh, tphl, and PWD) of the optocoupler can be improved.

At the moment of powering on or off the system, the I/O ports of some chips may be in an uncertain state, or even send out some erroneous pulses. The new generation of low-power optocouplers integrates a function similar to undervoltage lockout to avoid interference with the output when the power is lost or restored. This function is designed mainly to ensure that the output is in a certain state when the power is started or turned off without sending out false signals.

Design engineers usually face the problem of different communication wiring and different rise and fall time differences due to different loads. As can be seen in Figure 5, higher load capacitance will make the output rise and fall time longer, resulting in differences in transmission delay and pulse width distortion. Avago's new generation of optocouplers has added a slew rate controllable output to ensure that the output has stable rise and fall times under different load capacitances. This point is particularly important for parallel communication connections.

Figure 5: Problem of rise and fall time differences due to different loads

Digital communication systems face four circuit problems that affect reliability and performance, namely high voltage pulse transients, common mode noise caused by different ground transients, ground loops and level incompatibility. Ultra-low power optocouplers can not only save power, but also simplify the design of power supply and temperature management, improve LED life and directly connect to microcontrollers. Avago can provide low power isolation solutions without sacrificing high voltage insulation and noise isolation performance. In addition to changing the output polarity and increasing the speed by adding peaking capacitors, the use of separate balancing resistors also brings it excellent noise immunity.

Q&A Selection

Q: What is the difference between the principles of optocouplers in power drive circuits and optocouplers in zero-crossing detection circuits? How to choose optocouplers correctly?

A: From the perspective of optocoupler isolation, the two are similar. The optocoupler in the power drive circuit has a powerful current output function and can be used to drive IGBT or Power MOSFET. The optocoupler in the zero-crossing detection circuit may be a common HCPL-817 phototransistor, which has the function of signal output and cannot drive power devices.

Q: What is the difference in performance and reliability between ceramic package and plastic package optocouplers?

A: Plastic package optocouplers are sufficient for industrial projects. Ceramic packages are usually used in military or satellites.

Q: What is slew rate controlled output?

A: Slew rate controlled output: Basically, the output slew rate will change with the output load, because it takes more time to charge a higher load. Propagation delay and PWD will also increase with load capacitance, causing problems for parallel communication with different line load capacitances. Optocouplers have built-in slew rate control to ensure that the output has stable rise and fall times under different load capacitances.

Q: What should be paid attention to in the application of analog isolation amplifiers?

A: The main attention should be paid to providing stable power supplies on both sides of the analog isolation amplifier, wiring on the PCB board, and selecting the appropriate input signal range. Q

: In order to obtain better common-mode rejection characteristics, how should the pins of digital optocoupler isolators be set?

A: In order to ensure good common-mode rejection characteristics, LEDs with balanced input impedance (i.e. forward current and return current paths) can be input at both the anode and cathode. By using the "split resistor" method, the input impedance of each LED can be balanced. In this configuration, since the rise of the common-mode noise voltage at the LED input will be symmetrical, the LED cannot be turned on, thereby relatively independent of the input forward current.

Q: What factors are related to the response speed of optocoupler isolation? How high is it at present?

A: It is mainly related to the speed of LED, photoelectric sensor, etc. At present, it is as high as 50MB (HCPL-0710).

Q: Is it necessary to further shield the optocoupler externally? Are there any commercialized externally shielded optocouplers?

A: Unless it is in an extremely noisy or explosive environment, it may be necessary to attach a grounded shell to the PCB.

Q: In a digital switching power supply, how should the optocoupler be designed to achieve the requirements of precise voltage regulation?

A: Depending on the design requirements, a higher data rate may be required. The HCPL-0723 optocoupler can provide faster digital communication.