Keywords: controller driver semiconductor smart power supply

A typical programmable logic controller (PLC) today contains many analog and digital outputs used 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. Analog outputs present a special challenge (shown in Figure 1) because of the need to provide high-accuracy active drive setpoints over many different load conditions. The active driver stage becomes particularly important here; losses should be kept as small as possible.

The factors to consider are as follows:

connected load

Maximum allowable ambient temperature and internal module temperature

Number of channels and module size

Electrically isolated interface

precision

In process automation, it is also often necessary to establish electrical isolation between the individual output channels. Apart from that, there are some other requirements like channel-based diagnostics or HART? Signal support. Robustness and fault tolerance are also prerequisites.

figure 1. Block diagram of an 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 are possible. The functionality of the analog output channels can be fully integrated into the IC. Therefore, the AD5758 integrates the basic functions of a DAC and driver, as well as numerous other analog and logic functions, such as ADC for diagnostics, intelligent power management, voltage reference, protection against reverse and overshoot, in a 5 mm × 5 mm package size A high-voltage fault switch, a data calibration register, and an SPI communication interface.

The 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 subranges (such as for 4 mA to 20 mA for process automation). Each setting provides a 20% overrange range. These values are output in 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? Losses mainly occur in the power supply section with the DC-DC converter and output driver stage. This is where intelligent power management comes in. The AD5758 features adaptive load regulation or dynamic power control (DPC). The DPC is activated in current output mode and controls the voltage on the driver stage required to drive a specific load. Depending on the operating conditions, the load voltage (I × RLOAD) of the current output is only a small fraction of the supply voltage. The supply voltage difference has to be dissipated in the form of power losses beforehand through the series transistors. The DPC now regulates the driver voltage to a few volts above the actual desired load voltage (leaving headroom for the output transistors) to minimize losses. Effective voltage regulation in this way is only possible with a switching regulator, which is already integrated in the AD5758 and automatically controlled according to the load. Even with the additional losses in the switching regulator and upstream power supply, the reduction in overall power loss is still significant, especially for small load resistances (see Table 1). This enables small form factor designs in the first place, and the board also maintains good heat dissipation.

Derating sets strict limits

Derating is defined as a reduction in performance under specified boundary conditions, similar to the safe operating area (SOA) in power semiconductors. Output modules that do not employ DPC are subject to tighter thermal constraints due to the aforementioned power losses and associated cooling issues. Today, it is common to have two or four channels on a credit card-sized module. Modules are typically rated for an ambient temperature of up to 60°C. However, under these ambient conditions, not all four channels can drive very small loads, because in the four channels without DPC, the power loss in the module can reach 3 W, and the heat generated can quickly make the components reach its limit value. With thermal derating (Figure 3), module manufacturers can use only one or two of the four available channels at higher ambient temperatures, greatly reducing availability and channel cost performance.

image 3. Typical Derating Curve

Because of the AD5758's adaptive regulation, its power loss depends only on the load resistance to a small extent, and remains consistently below 250 mW for loads from 0 kΩ to 1 kΩ (Table 2). Therefore, depending on the design of the output module, eight isolated channels will be achieved with an overall power loss of <2 W. The 5 mm × 5 mm LFCSP package has a junction-to-ambient thermal resistance ΘJA of 46 K/W and a temperature rise of less than 10°C at a power loss of 200 mW. The AD5758 is rated for ambient temperatures up to 115 °C. This provides a lot of headroom for multi-channel modules without derating.

Table 2. Power measurement in DPC operating mode at I = 20 mA and supply = 24 V

The power dissipation value also includes the power dissipation due to the use of the ADP1031 for power and data isolation.

Power optimization

Supply voltages have different requirements:

Logic Voltage: In addition to the driver supply (operating mode depends on unipolar or bipolar), the AD5758 output IC requires a 3.3 V logic voltage to power the internal modules. This can be generated using an on-chip LDO regulator; however, to improve efficiency and reduce power loss, a switching regulator is recommended.

Isolated driver power supply: For safety reasons, electrical isolation is always maintained between the PLC bus and the I/O modules. Figure 1 shows this isolation in different colors, including three different potentials for the logic (bus) side, power supply, and field side outputs.

The isolation, power and output drivers are integrated into a single chip because the three parts are also typically spatially separated on the board, i.e. the outputs are located towards the front connector terminals, and the backplane bus (as the name implies) is on the back 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 reduced space requirements and power loss (Figure 5).

Figure 4. Power Management Unit ADP1031.

The ADP1031 integrates four modules in a 9 mm × 7 mm package size:

Flyback converter for generating positive isolated supply voltage VPOS.

Inverter to generate the negative supply VNEG required for bipolar output.

Step-down converter used to provide VLOG for the logic circuit of the AD5758.

Isolated SPI data interface with additional GPIO.

The advantage of a flyback converter is high efficiency; only a small 1:1 transformer is required. The flyback converter can generate isolated driver voltages up to 28 V in the first stage. This creates an inverter and a buck converter that share the same ground potential.

During the design of the power management unit, Analog Devices has made special efforts to enhance electromagnetic compatibility (EMC) and robustness. For example, the output voltage is phase shifted, and the slew rate of the flyback controller is adjustable. Soft-start, overvoltage protection, and current limit functions have also been added for all three voltages for good measurements.

Isolated SPI interface based on proven iCoupler? technology that transmits all the control signals needed for the job. A distinction is thus achieved between the high-speed data path (four lanes) and the lower-rate GPIO control path (three multiplexed lanes). Potential applications are the simultaneous activation of a multi-channel module or outputs in multiple modules via a common control signal, readback of error flags or triggering of a safe shutdown.

System advantage

The combination of the AD5758 and ADP1031 provides the full functionality of an isolated analog output with only two chips. Measuring approximately 13 mm × 25 mm, the aisle space requirement is smaller, half that of current solutions.

In addition to saving space, the integration of key functions results in a cleaner layout, easier separation of potentials, and a significant reduction in hardware costs. Analog Devices' 8-channel demo design uses only a six-layer board, measuring 77 mm × 86 mm (Figure 6).

Summary of advantages:

Smaller modules and more channels per module through power loss optimization

No derating required, allowing higher ambient temperature

Reduced hardware effort, thereby lowering costs

Easy scalability of multi-channel modules

Robust design and more diagnostics

Figure 5. A complete 4-channel analog output using the ADP1031 and AD5758

Image 6. Isolated 8-channel AO module