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Quad 16-Bit Voltage-/Current-Output DACs Save Space, Cost, and Power in Multichannel PLCs
The industry has a continuing tendency to increase the number of sensor and control nodes in the remote area, causing a corresponding increase in the number of I/O module nodes in the controllerand some distributed control systems (DCS) can handle thousands of nodes. This concentration of nodes brings increased temperature-related challenges, especially for systems that implement the 4-mA to 20-mA loop communications standard.
Perhaps the biggest and most relevant challenge to the system designer is the need for greater efficiency and reduced power consumption, as the inefficiency of existing systems results in wasted power and increased operating costs. This article explains the challenges of designing such systems for greater efficiency and introduces the AD5755, a versatile, 4-channel, 16-bit digital-to-analog converter (DAC) as a more integrated solution to help resolve these issues.
Figure 1. Hierarchy in a control system.
Some of the advantages of PROFINET are
At the field level, field bus protocols, used to interconnect industrial drives, motors, actuators, and controllers to the PLC/DCS I/O systems, are numerous, including DeviceNet,™ CAN, and InterBus,® as well as the above-mentioned PROFIBUS and Fieldbus.
An input-output (I/O) controller connects to sensors and control actuators in a factory- or process environment; it communicates with multiple end nodes by analog and digital means, as noted above. Intrinsically safe systems connect via 4-mA to 20-mA current loops, and some use isolation. The control processor is typically an 8-bit to 32-bit processor with performance of up to 100 DMIPS (Dhrystone millions of instructions per second). Factory automation equipment is ruggedly constructed for fanless operation in a harsh industrial environment.
Examples of 8-channel analog I/O modules are featured in Figure 2. Because of their small size, they have limited power-dissipation capability, some even less than 5 W.
Figure 2. I/O modules.
Analog 4-mA to 20-mA current loops are commonly used for signaling in industrial process control, with 4 mA representing the low end of the range and 20 mA the high end. The key advantages of the current loop are that accuracy of the signal is not affected by voltage drops in the interconnecting wiring and the loop can supply up to 4 mA for powering the device. Even if the line has significant electrical resistance, the current loop transmitter will maintain the proper current, up to its maximum voltage capability.
The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop (for example, 0 mA indicates an open loop, or 3 mA could indicate a fault condition on the sensor) and also allows 2-wire-transmitter devices to be powered by the loop current. Such instruments are used to measure pressure, temperature, flow, pH, and other process variables and to control a valve positioner or other output actuator. The current in an analog current loop can be converted to a voltage input at any point in the loop with a series precision resistor. Since input terminals of instruments may have one side of the current loop tied to the chassis ground (earth), analog isolators may be required when connecting several instruments in series.
P = V × I = 20 V × 0.02 A = 0.4 W.
If the load resistance was changed to 100 Ω, using the same supply (a valid condition), the power dissipated would still be 0.4 W, even though only 0.04 W is needed. In this case there is a 90% loss of efficiency in the system, with 360 mW being wasted.
Figure 3. Power is wasted when full-scale output is much less than the power-supply voltage.
With an 8-channel module, the total power dissipation with a 20 V supply would be 3.2 W, of which as much as 2.88 W would be wasted in the module (if all loads are 100 Ω). In such cases, self heating, as well as the effect of the increased power budget, starts to become a consideration. Increased temperatures within the module can lead to increased system errorsthe drift specs of the individual components need to be factored into the overall system error budget.
Designers may consider various ways to solve these problems:
In any event, the trend to provide an increased number of channels in a smaller space will cause further thermal power problems for many system designers.
One way to help solve this problem is to start with a 5-V supply. Monitor the output load voltage, then efficiently boost and regulate the output voltage as needed. The 5-V supply and an efficient dc-to-dc boost converter use feedback control to provide the appropriate output voltage, minimizing the on-chip power dissipation (Figure 4).
Figure 4. Dynamic supply control principle.
This kind of closed-loop dynamic power capability can be found in the AD5755 family of 4-channel, 16-bit, serial-input, voltage- and current-output DACs (see AppendixFigure A). Because each of its four channels can individually furnish either current or voltage with 16-bit resolution, with output powered by an individual dc-to-dc converter under dynamic power control, the device provides the equivalent of four low-dissipation nodes in a very compact 9-mm × 9-mm × 0.8-mm package.
The simplified circuit of Figure 5 shows how the dynamic power control works, using an inductive boost circuit. Each channel is capable of providing a boosted output voltage greater than 30 V. The dynamic power control mechanism uses feedback to regulate the output voltage, which is divided down by a resistive voltage divider and compared to the reference voltage in an internal error amplifier to create an error current. At the beginning of the switching cycle, the MOSFET switch is turned on and the inductor current ramps up. The MOSFET current, converted to a voltage, is measured. When the current-sense voltage is greater than the error voltage, the MOSFET is turned off and the inductor current ramps down until the internal clock initiates the next switching cycle. A similar scheme is used to regulate the output compliance voltage in current mode. In this case a feedback error current is used.
Figure 5. Voltage boost with power control.
The user has the option to switch the frequency and phase of each channel's dc-to-dc converter switching signals to allow for circuit and component optimization.
The dynamic power control on the output driver is designed to minimize package power dissipation. Typical ICs can operate at internal junction temperatures (TJMAX) up to 125°C. Assume the ambient temperature, TA, in the system is 85°C. The thermal impedance, θJA, for the LFCSP package is typically 28°C/W. To calculate the allowable on-chip dissipation we can use the following analysis.
Without dynamic power control, assuming a 24-V supply, the worst-case power dissipation (per channel) can be calculated to be
Four channels would dissipate nearly 2 W under similar conditions; this would cause problems for both the module and the semiconductor circuitry. By enabling the dynamic power feature, the AD5755 regulates the supply to minimize the on-chip power dissipation. Figure 6 shows a comparison of the power dissipated per channel with dynamic power enabled and disabled (fixed supply).
Figure 6. Dissipation comparison with and without dynamic power control.
When the dynamic power capability is enabled, the on-chip power dissipation is about 50 mW with output current of 24 mA vs. 400 mW with no regulation. This ability to control the on-chip power dissipation is of great value to the system designer because the number of channels in the system can be increased while minimizing module dissipation. It thus eliminates the need to consider extensive (and expensive) methods to control system temperatures.
Checking and Diagnostics Under Fault Conditions:
One serious consideration is where the MCU/DSP that controls the DAC goes when a fault condition occurs. With no ability to control the output, the user would lose complete control of the system. The AD5755 has a watchdog timer (with programmable timeouts) that sets an alert flag (active high) if it has not received a command over the SPI interface within the timeout period. If desired, this ALERT pin can be directly connected to the clear pin (also active high) to set the outputs into a known safe condition (Figure 7). Each channel on the AD5755 has a 16-bit programmable clear code register, giving the user flexibility to clear the output to any code.
Figure 7. Watchdog timer flags loss of control signal and returns DAC to clear setting.
Even with the MCU operating normally, communications signals can become corrupted in noisy industrial environments. For dealing with this possibility, the AD5755 has an optional packet error-checking (PEC) function, which implements a CRC8 polynomial routine. This can be enabled or disabled through software to ensure that the output is never incorrectly updated.
Miswiring on the output can often lead to open- or short-circuit connections, potentially damaging the system. (Even if no damage occurs, the problem can often be difficult to diagnose. The AD5755 has open- and short-circuit detection, immediately setting a fault flag to alert a technician of the problem). In addition, short-circuit protection limits the output current in the event of a short circuit. All faults can be communicated via the SPI interface or through a hardware fault pin, allowing the user to take immediate action.
Figure 8. Arbitrary range scaling.
Additional Information Over the 4-mA to 20-mA Current Loop
HART provides for a digital two-way communication scheme that is compatible with 4-mA to 20 mA current loops. A 1-mA peak-to-peak frequency-shift-keyed (FSK) signal is superimposed on the 4-mA to 20-mA analog current signal. The two frequencies used are 1200 Hz (Logic 1) and 2200 Hz (Logic 0), based on the BELL 202 communications standard (Figure 9).
Figure 9. HART signal riding on an increasing loop current.
The AD5755 can be configured to transmit a HART signal with only two external components. The output of the HART modem is attenuated and ac-coupled at the CHART pin of the AD5755; this results in the modem output being modulated on top of the 4-mA to 20-mA analog current without affecting the "dc" level of the current. The circuit in Figure 10 shows how the AD5755 can interface to a HART modem to embody this dual form of communication.
Figure 10. The AD5755 in HART communication.
The HART specification requires that the maximum rate of change of analog current not interfere with HART communications. Obviously, step changes in the current output can disrupt HART signaling. Fortunately, the AD5755 has controllable slew rate, which, when enabled, allows the user to digitally limit the slew rate of the current output.
Figure 11. AD5755 setup.
As a 4-channel device in a 9-mm × 9-mm CSP package, the AD5755 dramatically helps reduce board area while increasing channel density. With dynamic power control, the on-chip power dissipation is regulated and module power dissipation is minimized. The addition of on-chip diagnostics, including watchdog timers, PEC error checking, and open-/short-circuit detection and protection gives the end user higher confidence that the robust design is capable of working in harsh industrial environments. The AD5755 is a true system-on-a-chip solution.
The AD5755 uses a versatile 3-wire serial interface that operates at clock rates up to 30 MHz and is compatible with standard SPI,® QSPI,™ MICROWIRE,™ DSP, and microcontroller interface standards. The interface also features optional CRC-8 packet error checking, as well as a watchdog timer that monitors activity on the interface.
The AD5755 feature 16-bit resolution and monotonicity, voltage or current output on the same pin, user-programmable offset and gain, on-chip diagnostics, an on-chip 5 ppm/°C max voltage reference, and a 40°C to +105°C operating temperature range. Available current-output ranges are 0 mA to 20 mA, 4 mA to 20 mA, and 0 mA to 24 mA ±0.05%; available voltage ranges are 0 V to 5 V, 0 V to 10 V, ±5 V, ±10 V, ±6 V, and ±12 V ±0.05%. (return to text)
Figure A. Functional block diagram of the AD5755 quad DAC. All four channels are identical.
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