“For a long time, isolation has been regarded as an indispensable burden by designers. It is essential because it can make Electronic components safe so that anyone can use it. It is a burden because it limits the communication speed, consumes a lot of power, and takes up a lot of board space. Optocouplers based on old technology, and even many newer digital isolators, have very high power consumption, rendering certain types of applications unfeasible. In this article, we will examine the latest developments in the field of ultra-low power isolation, its relationship with existing technologies, and its implementation.At the same time, we will also explore
For a long time, isolation has been regarded as an indispensable burden by designers. It is essential because it can make electronic components safe so that anyone can use it. It is a burden because it limits the communication speed, consumes a lot of power, and takes up a lot of board space. Optocouplers based on old technology, and even many newer digital isolators, have very high power consumption, rendering certain types of applications unfeasible. In this article, we will examine the latest developments in the field of ultra-low power isolation, its relationship with existing technologies, and its implementation. At the same time, we will also explore a variety of applications that can benefit from such new devices.
For designers, the modern optocouplers that appeared about 45 years ago are a huge improvement. They allow feedback in power control circuits, signal isolation in communication circuits to interrupt ground loops, and communication with high-end power transistors or current monitors.
In the 1970s, a large number of optoelectronic devices emerged. These devices have influenced the development of communication standards such as RS-232 and RS-485, as well as the development of 4 to 20 mA circuit loops and industrial buses such as DeviceNet and PROFIBUS. Affected by the limitation of the isolation device itself, the function of optical isolation determines many characteristics of these communication buses. In the next 20 years, the development and changes of isolation technology were basically quantitative changes, and by 2000, the first batch of new chip-level digital isolators appeared on the market. These new devices are based on inductive coupling technology, using chip-scale transformers, GMR materials and later differential capacitive coupling technology. Compared with older optocouplers, these new technologies can achieve ultra-high speed and ultra-low power consumption levels. However, due to the limitations of the standards implemented at the time, many functions (such as high speed) of the new devices were not fully available. Use, because the current standard interface does not need these functions.
After the digital isolator adopts standard packaging and IC technology to manufacture its encoding and decoding electronic components, the addition of digital functions becomes very simple. Low power consumption, support for low power supply voltage, and high integration have become the main design advantages of non-optical isolators. New technologies that can greatly increase the isolation rate and greatly reduce the isolation power consumption can support the most demanding new interface standards. At present, the power consumption of digital isolators (much lower than optocouplers) needs to be two to three orders of magnitude lower in order to enter the new application space. So far, high-performance isolation has not been able to achieve this goal.
Comparison of various technologies
The rapid development of isolation device performance is the result of the combined effect of the data encoding scheme and the efficiency of the medium used for data transmission. In this article, we will focus on the various aspects that determine power consumption. Encoding and decoding schemes can be roughly divided into edge-encoded pulse-based systems and level-encoded systems. Simply put, a level-based system must continuously push energy across the isolation barrier to maintain an active output state, while at the same time, it expresses a passive output state by not sending energy across the isolation barrier.
In an optical coupler, light mediates energy transmission. Compared with directly establishing an electric or magnetic field, its efficiency is lower, and at the receiving element end, its detection efficiency is poor. Therefore, a simple transistor or PIN diode-based optocoupler needs to consume a lot of power to generate light to keep the input on, but the receiver only needs to consume very little power to receive the signal. This can be seen in Table 1, which lists the power consumption of the PIN diode receiver optocoupler. On average, this type of optocoupler has the characteristics of high input current and low output current. The higher-speed digital optical coupler reduces the amount of light required to maintain a certain state by adding an active amplifier module in the receiver. This reduces the average current required by the LED, but the receiver has a relatively large quiescent current, so its power consumption is not really reduced-it is just pushed to the receiver. Reducing the required power consumption requires increasing the efficiency of the LED and receiver components, or changing the coding scheme. This is the reason why optocoupler technology has only achieved quantitative development in such a long period of time.
In many capacitively coupled digital isolators, the system is actually similar to an optocoupler. This type of device uses a high-frequency oscillator to transmit the signal through a pair of differential capacitors. This oscillator, much like an LED in an optocoupler, needs to consume power to send an active state, and shut down to send a passive state. The receiver is equipped with an active amplifier, which consumes bias current in both states. As shown in Table 1, due to the higher coupling efficiency of the capacitor, the total power consumption is significantly better than the optocoupler option. It should be noted that if inductive coupling rather than capacitive coupling technology is used, the power level of the digital isolator is roughly equivalent. In this case, it is the coding scheme that determines the lowest power level, especially at low data rates.
ADI’s iCoupler-type digital isolators (such as the ADuM140x series) use another coding scheme, as shown in Figure 1. In this scheme, edges are detected at the input and encoded into pulses. In ADuM140x, one pulse represents a falling edge, and two pulses represent a rising edge. These pulses are coupled to the secondary winding through a small on-chip pulse transformer. The receiver counts the pulses and reconstructs the data stream. The pulse itself has excellent robustness and an excellent signal-to-noise ratio can be obtained, but its width is only 1ns. Therefore, the energy of each pulse is very low. The result is a very good property, that is, when there is no data change, the state of the output terminal will be latched and maintained, and almost no power is consumed. This means that the power consumption is the integrated electric energy transmitted in the pulse stream plus a certain bias current. As the data rate drops, power consumption decreases linearly until DC. Similarly, it is the encoding scheme that leads to the reduction in power consumption, not the specific data transmission medium, which can be implemented in capacitive or even optical systems.
Figure 1. Pulse-based coding scheme
Pulse coding schemes are not a panacea for low power consumption. The disadvantage is that if there is no logical change at the input, the data will not be sent to the output. This means that if there is a difference in DC level due to the start-up sequence, the input and output will not match. The ADuM140x solves this problem by implementing a refresh watchdog timer on the input channel. If no activity is detected for more than 1μs, the DC status will be retransmitted. The result of this design is that when the data rate is lower than 1 Mbps, the encoding scheme no longer continues to reduce power consumption. The device basically always runs at a rate of at least 1Mbps, so at low data rates, power consumption will not continue to drop. Even so, compared with the level-sensitive scheme shown in Table 1, the average power consumption of the pulse coding scheme is lower.
Table 1. Comparison of the power consumption of each channel of the isolator (VDD=3.3V, 100kbps)
Further reduce power consumption
The ADuM140x pulse coding scheme was originally optimized for high data rates rather than absolute minimum power consumption. The coding scheme has great potential in reducing power consumption, especially in the frequency range of DC to 1Mbps. This data range is the range used by most isolation applications, especially isolation applications that require low power consumption. The series of devices based on 4-channel ADuM144x and 2-channel ADuM124xiCoupler technology use the following innovative technologies.
1. The design is implemented using a lower voltage CMOS process
2. All bias currents are evaluated, and the bias is minimized or eliminated as much as possible
3. Reduce the frequency of refresh current from 1MHz to 17kHz
4. The refresh circuit can be completely disabled to achieve the lowest power consumption
Power consumption is a function of frequency, as shown in Figure 2 (compared to ADuM140x). For ADuM140x, the curve “knee” caused by refresh is clearly visible at 1Mbps, and for ADuM144x, when refresh is enabled, it is clearly visible at 17kbps. The typical power consumption per channel of the ADuM144x is 65 times lower at 1kbps, and about 1000 times lower when the refresh function is completely disabled.
Figure 2. Total power consumption per channel for ADuM144x and ADuM140x devices under VDDX=3.3V
Why is it useful to drop so much power consumption? In the following three applications, traditional optocouplers and digital isolators are either barely qualified or completely unusable.
4mA to 20mA isolated loop-powered field instrument
Figure 3. Isolated, loop-powered smart sensor front end with HART modem support
The power budget of loop-powered field instruments is very limited, because all electrical energy comes from the 4mA loop current. Fortunately, the loop can usually provide enough voltage, usually 24V, to get about 100mW of power from the system. The entire application will consume approximately 12V loop voltage (4mA). Within this budget, a simple DC-DC converter powers isolated sensors, analog-to-digital converters (ADC), and controllers. Even assuming that the DC-DC converter has higher efficiency and the voltage step-down ratio is 2:1, the power that a typical sensor front end can provide is less than 4mA (3.3V). The power budget at the loop end is roughly the same. The main interface is the SPI bus connected to the ADC. Each end of the isolation interface is powered by the loop, and all ADCs and signal conditioning components of the controller are powered by the loop. Table 2 shows the power consumption of a 4-wire SPI bus under each isolation technology. SPI1 is the isolated loop terminal current, and SPI2 is the required sensor terminal current. The optocoupler will consume multiple times the power budget at each end of the isolation interface. Capacitive digital isolators will consume the entire power budget of the field instrument. The ADuM1401 represents a possibility, but the power budget for the rest of the system is very narrow, even if it only supports a single SPI interface to the ADC. The ADuM1441, an ultra-low-power digital isolator with iCoupler technology, has very low power consumption and only accounts for a small part of the power budget. This technology not only allows the application to work normally within its power budget, but also allows the addition of a second 4-channel isolator to support the HART modem interface and intelligent front-end controller, as shown by the dotted line in the figure. The iCoupler technology with ultra-low power consumption can realize new functions that were not possible in previous isolation applications.
Table 2. The total power consumption of each end of a 100kbps isolated SPI interface
Power over Ethernet I2C communication bus
Figure 4. POE, 4-port controller with isolated I2C and interrupt
Telecommunications applications such as Power over Ethernet (POE) derive power from a relatively high voltage rail, which provides Ethernet power. The control communication interface must obtain power from an isolated DC-DC converter or through a C54V bus voltage regulator. In the example shown in Figure 4, the 3.3V I2C control bus communication interface is generated by the built-in voltage regulator in the POE controller. Figure 3 shows the current required to run the I2C bus interface on the POE controller, and the power consumption of the POE controller to support each technology. The optocoupler solution generates half a watt of heat in the chip, which is likely to be close to its thermal limit. In the table, from top to bottom, each interface is slightly better than the previous one, and finally we see the ultra-low power consumption ADuM1441, which consumes about 1mW. As a result, the thermal load of the interface appears insignificant in this chip. Even if the power supply is not adjusted inside the POE chip, the power consumption is very low. A simple Zener diode and resistor can be used, so that the cost of energy-saving components and the cooling load can reach a reasonable level. This technology simplifies the power architecture.
Table 3. Total power consumption of various isolation technologies in POE applications
Figure 5. Battery-powered medical sensors
The third application example of ultra-low power consumption is to provide support for longer battery-powered applications. Medical equipment for home health monitoring (such as blood glucose meters, pulse oximeters) must adopt a special structure so that it can be connected to non-medical computers while contacting patients. The serial interface must be powered and the device can be awakened when connected to the computer. Therefore, an active isolator should be used in the standby circuit. In this case, using the refresh disable function of the ADuM1441 can reduce the battery power consumption of the device to below 4μA. This level of power consumption is very low, even a button battery can maintain the standby current for several years.
The ultra-low power consumption of ADuM1441 also supports convenient power supply for the end of the isolation module facing the computer. Only a few μA current is needed to realize the interface operation. Therefore, a status line in the serial interface can be used exclusively for powering the isolator, so there is no need to use a special power supply.
Table 4 shows some of the attributes of the optocoupler and various digital isolations that work in standby mode. Please note that if the correct idle state is selected, the standby current of the PIN/transistor isolator may actually be as low as a product based on the ultra-low power iCoupler. People take advantage of this feature of optocouplers to achieve low-power standby in many applications. However, once communication is started, power consumption will rise to a higher level, which is not the case with the ADuM1441 solution.
Table 4. Total low-speed and idle power consumption of the isolator
ADI has developed a new version of the pulse-coded iCoupler digital isolator, which is optimized for extremely low power consumption. The changes made to the device did not affect the isolation function of the device, because the insulation technology used is exactly the same as in the high-resolution reinforced insulation device. The signal integrity is similar to the standard iCoupler that has appeared on the market for the past 13 years. According to the design, these devices can support ultra-low power operation in the DC to 1Mbps range. The lower the data rate, the lower the power consumption. This technology has much lower operating power consumption, so it can achieve interface isolation performance that was previously impossible.