Today’s typical programmable logic controller (PLC) contains many analog and digital outputs to control and monitor industrial and production processes. Modularity is widely adopted, and in terms of input and output (I/O), it covers the basic functions of analog I/O and digital I/O. The analog output presents a special challenge (as shown in Figure 1) because it is necessary to provide high-precision active drive settings under many different load conditions. The active driver stage becomes especially important at this time; losses should be as small as possible.
The factors to be considered are as follows:
·Maximum allowable ambient temperature and internal module temperature
·Number of channels and module size
·Electrical isolation interface
In process automation, it is usually necessary to establish electrical isolation between each output channel. In addition, there are some other requirements, such as channel-based diagnostics or support for HART? signals. Robustness and fault tolerance are also necessary conditions.
Figure 1. Block diagram of isolated analog output system.
Due to the development of semiconductors and the continuous improvement of mixed-signal technology, ultra-small circuits with high integration density become possible. The function of the analog output channel can be completely integrated into the IC. Therefore, the AD5758 integrates the basic functions of the DAC and driver, as well as many other analog and logic functions, such as ADC for diagnostics, intelligent power management, voltage reference, and protection against reverse and overshoot in a package size of 5 mm × 5 mm. Voltage fault switch, data calibration register and SPI communication interface.
AD5758 (Figure 2) covers all common output ranges used in automation: unipolar 0 V to 10 V/0 mA to 20 mA, bipolar ±10 V/±20 mA, and all sub-ranges (for example, for 4 mA to 20 mA for process automation). Each setting provides an over-range range of 20%. The output of these values uses 16-bit resolution.
Figure 2. Functional block diagram of AD5758.
Power loss is greatly reduced
What properties make the AD5758 particularly suitable for temperature and space-constrained applications? The loss mainly occurs in the power supply part with DC-DC converter and output driver stage. This is where smart power management comes in. AD5758 has adaptive load regulation or dynamic power control (DPC) function. The DPC is activated in the current output mode and controls the voltage on the driver stage required to drive a specific load. According to the operating conditions, the load voltage (I × RLOAD) of the current output only accounts for a small part of the power supply voltage. The power supply voltage difference must be dissipated in the form of power loss through series transistors in advance. The DPC now regulates the driver voltage to a few volts higher than the actual load voltage required (reserving margin for the output transistor), thereby minimizing losses. Only by using a switching regulator can the voltage be effectively regulated in this way, and this device has been integrated in the AD5758 and can be automatically controlled according to the load. Even if additional losses occur in the switching regulator and the upstream power supply, the overall power loss reduction is still very significant, especially for small load resistances (see Table 1). This first makes it possible to design a small size, and the circuit board can also maintain good heat dissipation.
Table 1. Theoretical loss when the output current I = 20 mA and the fixed power supply voltage is 24 V (not considering the internal power consumption and efficiency of DC-DC)
Set strict limits for derating
Derating is defined as performance degradation under specified boundary conditions, similar to the safe operating area (SOA) in power semiconductors. Due to the aforementioned power loss and related cooling issues, output modules that do not use DPC are subject to stricter thermal restrictions. Nowadays, it is common to have two or four channels on a module the size of a credit card. Usually the rated ambient temperature of the module is up to 60°C. However, under these environmental conditions, not all four channels can drive very small loads, because in the four channels that do not use DPC, the power loss in the module will reach 3 W, and the heat generated will make the components quickly reach Its limit value. Through thermal derating (Figure 3), module manufacturers can only use one or two of the four available channels at higher ambient temperatures, thereby greatly reducing availability and channel cost performance.
Figure 3. Typical derating curve.
Because the AD5758 has an adaptive adjustment function, its power loss depends only on the load resistance to a very low extent. For a load of 0 kΩ to 1 kΩ, its power loss is always kept below 250 mW (Table 2). Therefore, according to the design of the output module, eight isolated channels can be realized, with a total power loss of <2 w. the junction-to-ambient thermal resistance Θja of 5 mm × lfcsp package is 46 k>
Table 2. Power measurement values in DPC working mode when I = 20 mA and power supply = 24 V
The power supply voltage has different requirements:
Logic voltage: In addition to the driver power supply (operating mode depends on unipolar or bipolar), the AD5758 output IC also requires a 3.3 V logic voltage to power the internal modules. This can be generated using an on-chip LDO regulator; however, in order to improve efficiency and reduce power loss, it is recommended to use a switching regulator.
·Isolated driver power supply: For safety reasons, electrical isolation is always maintained between the PLC bus and the I/O module. Figure 1 shows this isolation in different colors, including three different potentials output from the logic (bus) end, the power supply, and the field end.
Because these three parts are usually spaced apart on the circuit board, that is, the output terminal is set toward the front connector terminal, and the backplane bus (as the name suggests) is located on the back, so the isolation, power supply and output driver are integrated into a single chip Not wise.
The power management unit ADP1031 (Figure 4) performs all functions and works in conjunction with the AD5758, enabling the development of isolated output modules with smaller space requirements and power consumption (Figure 5).
Figure 4. Power management unit ADP1031.
The ADP1031 integrates four modules in a 9 mm × 7 mm package size:
·Flyback converter, used to generate positive isolation power supply voltage VPOS.
·Inverter, used to generate the negative power VNEG required for bipolar output.
Step-down converter, used to provide VLOG for AD5758 logic circuit.
· Isolated SPI data interface with additional GPIO.
The advantage of the flyback converter is its high efficiency; only a small 1:1 transformer is required. The flyback converter can generate an isolated driver voltage of up to 28 V in the first stage. This generates an inverter and a buck converter, which share the same ground potential.
In the design process of the power management unit, ADI has especially strengthened electromagnetic compatibility (EMC) and robustness. For example, the output voltage is phase-shifted, and the slew rate of the flyback controller is adjustable. At the same time, soft-start, overvoltage protection, and current limit functions have been added to all three voltages to achieve good measurements.
The isolated SPI interface is based on mature iCoupler™ technology and can transmit all control signals required for work. Therefore, the distinction between the high-speed data path (four channels) and the lower rate GPIO control path (three multiplexed channels) is realized. Potential applications are to activate multi-channel modules or outputs in multiple modules synchronously through a common control signal, read back error flags, or trigger a safety shutdown.
The combination of AD5758 and ADP1031 provides the complete function of isolated analog output, requiring only two chips. The size is about 13 mm × 25 mm, and the channel space requirement is smaller, which is only half of the current solution.
In addition to saving space, the integration of key functions also makes the layout more concise, the potential is easy to separate, and the hardware cost is significantly reduced. ADI’s 8-channel demo design uses only a six-layer board, which measures 77 mm × 86 mm (Figure 6).
Summary of advantages:
·Through power loss optimization, make the module smaller and each module has more channels
·No need to derate, allowing higher ambient temperature
·Reduce hardware workload, thereby reducing costs
·Easily realize the scalability of multi-channel modules
· Reliable design and more diagnostic functions
Figure 5. Use ADP1031 and AD5758 to achieve a complete 4-channel analog output.
Figure 6. Isolated 8-channel AO module.
About the Author
Jürgen Schemel is currently a field application engineer at ADI, providing support to industrial strategic customers in the fields of automation, industry 4.0 and condition monitoring applications. He received a master’s degree from Offenburg University of Applied Sciences in 1996. He first worked at Siemens, engaged in the design of communication technology systems for industrial applications.