[Introduction]Automation control is increasingly common in industrial and consumer applications, but even best-in-class automation solutions rely on an ancient technology: the current loop. Current loops are ubiquitous components in control loops that work in both directions: they pass measurements from sensors to a programmable logic controller (PLC), and vice versa, from the PLC to process modulation devices.

A 4 mA to 20 mA current loop is the prevailing industry standard method for accurate and reliable data transfer from remote sensors to PLCs over twisted pair wires. Simple, durable, robust, proven and reliable long-distance data transmission, good noise immunity and low installation costs make this interface ideal for long-term industrial process control and automatic monitoring of remote objects in noisy environments. Traditionally, power for the current loop has been provided through a linear regulator for many of the reasons mentioned earlier. The disadvantages of using linear regulators are their relatively low efficiency and limited current capacity compared to switching regulators. Inefficiencies can lead to thermal issues, and limited current often prevents adding required control system functions.

The new high-efficiency, high-input-voltage buck regulators are robust and small enough to replace linear regulators in many current-loop systems. Buck regulators have many advantages over linear regulators, including higher current capacity, wider input range, and higher system efficiency. Buck regulators offer significant performance advantages, t at high switching frequencies_ONShorter times help provide a compact, robust solution.

background knowledge

The standard 4 mA to 20 mA current loop shown in Figure 1 can be used to transmit sensor information and control signals from field instrumentation to process modulation devices such as valve positioners or other output actuators. It consists of four parts:

● Current loop power supply: supply voltage V_DCVaries according to application (9 V_DC12 V_DC24 V_DCetc.), the potential is at least 10% higher than the voltage drop across the combined components in the circuit, such as the transmitter, receiver, and wires. This VDC is tapped by a local buck regulator to power sensors and other components.

● Transmitter: The main part of the transmitter is the sensor or transducer. It converts physical signals such as temperature, pressure, current, distance or magnetic fields into electrical signals. If the converted signal is an analog voltage, a voltage-to-current converter is required as part of the transmitter to convert the voltage to a 4 mA to 20 mA current signal. For smart digital output sensors, the digital signal is converted back to an analog signal by a DAC. Local power supplies in transmitter LDOs or buck regulators power all of these analog, digital, and reference circuits.

● Receiver or Monitor: The receiver converts the 4 mA to 20 mA current signal to a voltage signal that can be further processed and/or displayed. The current signal passes through a high precision shunt resistor R_SHUNTand/or analog-to-digital converters or data acquisition circuits to convert to useful voltage levels. At the meter terminal, a local buck regulator powers the receiver circuit.

● 2-wire or 4-wire loop: The complete current loop circuit can extend over 2000 feet and consists of a transmitter, power supply and receiver in series. In a 2-wire 4 mA to 20 mA current loop, the power supply shares the same loop as the current loop.

Figure 1.2 Schematic diagram of the wire current loop.

For example, to measure pressure from 0 psi to 50 psi using a remote pressure sensor, then a 4 mA to 20 mA current receiver circuit is placed in series with a pressure-to-current transducer. On the sensor end, it reads 4 mA at 0 psi and 20 mA at 50 psi. On the receiver side, from Kirchhoff’s first law, the same current will appear across the shunt resistor and convert it to a voltage signal.

Automated operations in industrial, refinery, highway monitoring, and consumer applications require high-performance sensor technology and reliable, accurate current loops to transmit sensor information. The components of the current loop must maintain high accuracy, low power consumption, and reliable operation over the extended –40°C to +105°C industrial temperature range, with the necessary safety and system functionality.

The supply voltage on the transmitter (sensor) side can be as high as 65 V during transients and must be converted to 5 V or 3.3 V. Since the sensor circuit is usually designed to take power directly from the current loop (no additional local power supply), it is usually limited to 3.5 mA. As transmitter functionality increases, this limitation becomes a problem when using traditional linear regulators, which cannot supply any additional current. Additionally, in systems using linear regulators, most of the power must be dissipated in the regulator, creating a lot of heat in the packaged system.

The LT8618 extends the input range to 65 V and the load capability to 15 mA. Its high efficiency removes thermal constraints in current loop system designs where the transmitter is packaged and exposed to harsh environmental changes. A low-cost filter is recommended to reduce voltage ripple and current ripple on the cable side. This article analyzes the performance of power regulators and provides component selection guidelines to meet stringent industry requirements. In addition, test data such as efficiency, startup, and ripple are provided.

Closing the Current Loop Using a Buck Converter with Extended Input and Load Range

The LT8618 is a compact buck converter with numerous features to meet the requirements of industrial, automotive and other unpredictable power environments. Ideal for 4 mA to 20 mA current loop applications, it features ultra-low quiescent current, high efficiency, wide input range, up to 65 V, and compact size. Figure 2 shows a complete transmitter circuit solution that uses the LT8618 to power the MAX6192C precision voltage reference, voltage-to-current conversion, and other circuits.

The current of the shunt circuit 2SC1623 is proportional to the voltage applied to the positive input of the error amplifier (EA). The 2.5 V reference is generated by the MAX6192C. The MAX6192C is a precision voltage reference IC featuring low noise, low dropout, and low temperature drift of 5 ppm/°C max. For smart sensors whose digital output is proportional to environmental variables, a DAC can convert the digital signal to an analog signal and send it to an error amplifier.

Therefore, with EA, BJT (2SC1623) and a 100Ω (±0.1%) sense resistor (RSENSE), the converter can modulate the current in the current loop from 4 mA to 20 mA, where 4 mA represents a non-zero minimum output and 20 mA mA represents the maximum signal. Even if the field transmitter has no process signal output, the 4 mA non-zero minimum output or output above zero can power the device. Therefore, the current in a shunt circuit is proportional to environmental variables such as pressure, temperature, level, flow, humidity, radiation, pH, or other process variables.

The two long wires are part of the information-carrying current loop and are also used from V_DC(power supply on the receiver side) to supply power to the transmitter. V_DCThe minimum voltage should be sufficient to cover the voltage drop between the conductors, shunts and the minimum operating voltage of the transmitter. The supply voltage depends on the application and is typically 12 V or 24 V, but can be as high as 36V.

At the remote transmitter terminal, a Schottky diode (D1) protects the transmitter from reverse current flow. Placing a Zener or TVS (D2) diode at the input provides further protection, limiting transient voltage surges proportional to the current loop inductance. The LT8618 high-efficiency monolithic buck regulator steps down the loop voltage to 5.5 V or 3.3 V to power the reference, DAC, and other functional units.

In Figure 2, V_DCThe wiring to the transmitter can vary from a few feet to 2000 feet. The stray inductance of the current loop forms an LC tank with the input capacitance of the buck regulator. Power side (V_DC) transients also appear on the input side of the remote transmitter. For worst-case undamped oscillation, the peak voltage may be V_DCtwice. For example, if the typical operating input voltage is 24 V and the maximum specification is 36 V, then the maximum voltage on the transmitter side may exceed 65 V. As shown in Figure 2, protection can be easily achieved using TVS diode D2 in front of the transmitter to limit any surges during transients.

Figure 2. Current loop with LT8618 as DC power supply.

Alternatively, an efficient system can be built by using an LDO regulator to protect the LT8618 from high voltage excursions. In this topology, the LDO regulator will regulate to the input voltage minus its dropout, while the LT8618 converts ~24 V to 5 V or 3.3 V with high efficiency. The current limit of the LDO regulator should be set below the usual 3.8 mA while maintaining high efficiency, and the input capacitors of the LT8618 basically use decoupling and storage capacitors. This will support brief bursts of high loads on the back end with minimal or no current loop current consumption. Because the high-voltage offset is relatively short and typically carries less total energy, the power losses that occur in the LDO regulator during these transients do not affect the overall efficiency; that is, the LDO regulator spends almost all of its time in high buck ratio.

A typical current loop will limit the input current of the power supply circuit that powers the entire remote transmitter, and the available load current of the LDO regulator cannot exceed this input current limit. On the other hand, a buck regulator can multiply the input current supplied to the load. Figure 3 shows the output current of the LT8618 regulator versus input current when converting from a 24 V input voltage to a 5.5 V output. For an input current limit of 3.8 mA, the output current is almost 15 mA. This extra power increases operating margin and enables additional functional units, simplifying the system designer’s job.

Figure 3. Output Current vs Input Current, V_IN= 24 V, V_OUT = 5.5V

Burst Mode Operation Improves Efficiency at Light Loads

Efficiency vs. Step-down Ratio of LDO Regulators (V_OUT/V_IN), when the input voltage is slightly higher than the output voltage, the efficiency will be high. The problem arises when the step-down ratio is high, when the efficiency is very low, which can cause a lot of thermal stress to the system. For example, when the input voltage is 55 V and the output voltage is 3.3 V, the power loss of the LDO regulator is 0.19 W and the load current is 3.8 mA. In contrast, a properly designed buck regulator can be very efficient at high step-down ratios. In addition, synchronous buck regulators can replace freewheeling diodes with MOSFETs to improve efficiency compared to non-synchronous regulators. The challenge for synchronous buck converters is to optimize efficiency over the entire load range, especially at light loads of 3 mA to 15 mA, where input voltages can be as high as 65 V.

For a typical synchronous buck converter, there are three main power losses: switching losses, gate drive losses, and losses associated with the converter IC controller logic circuitry. Switching and gate drive losses can be greatly reduced if the switching frequency is reduced, so simply running the converter at a low frequency reduces switching and gate losses at light loads.

At light loads, the bias losses of the logic circuit are comparable to the relatively low switching-related losses. The bias circuit is usually powered from the output and only takes power from the input through the internal LDO regulator during startup and other transient conditions. The LT8618 addresses logic circuit losses by operating in Burst Mode®. At this point, the current is delivered to the output capacitor in short pulses, followed by a relatively long sleep period during which most of the logic control circuits are turned off.

To improve light-load efficiency, a larger value Inductor can be chosen because more energy can be delivered to the output during short switching pulses, and the buck regulator can remain in sleep mode longer between these pulses. By maximizing the time between pulses and minimizing switching losses per short pulse, the LT8618 can deliver quiescent current below 2.5 μA while maintaining a regulated output at input voltages up to 60 V. Since many transmitter circuits have low current most of the time, this low quiescent current saves a lot of energy compared to typical buck regulators that consume tens or hundreds of μA of current.

Figure 4 shows the efficiency of the current loop solution shown in Figure 2 with 5.5 V_OUTThe output rail is connected to the BIAS pin of the LT8618. At 100 mA full load, peak efficiency reaches 87% with an input voltage of 28 V and an inductance of 82 µH. Under the same 28 V input voltage, the efficiency can reach or exceed 77% at 10 mA load, which is excellent.

Figure 4. LT8618 High Efficiency at Light Load, V_IN = 28 V, V_OUT = 5.5 V, L = 82 µH.

Input filter to limit inrush current and current loop ripple

The input of the power regulator is connected to the current loop, so in addition to steady-state current limiting, it is important to limit ripple and inrush currents during startup or load transients. The inrush current during power converter startup depends on the size of the input and output capacitors for a given soft-start time. This requires a trade-off: minimize the input capacitance to prevent large inrush currents, while making it large enough to maintain acceptably low ripple.

The input current of a buck converter is a pulsed current, so the input capacitor plays a key role in providing a filtering path for the ripple current. Without this capacitor, a large amount of ripple current would flow through the long current loop, resulting in unpredictable buck behavior. Therefore, there should be a minimum input capacitor to meet the ripple current and ripple voltage requirements. Multilayer Ceramic Capacitors (MLCCs) excel in ripple current due to their low ESR and ESL.

When the converter operates in burst mode, the inductor current follows a triangular waveform. The impedance of the current loop is much higher than the input filter. Therefore, the ripple voltage on the input capacitor can be estimated by the following equation, ignoring the ESR and ESL of the capacitor, where I_PEAKis the inrush current in the buck inductor, V_Ris the ripple voltage on the input capacitor (obviously, higher inrush currents require larger capacitors):

To minimize input voltage ripple while keeping the input capacitance as small as possible, we tend to use smaller buck inductors. However, burst mode is more efficient when using large inductors. With an 82 µH inductor and 1 V ripple, to avoid triggering the UVLO at any minimum input, a 100 nF input capacitor is sufficient for applications using the LT8618.

Most of the ripple current goes through the local decoupling capacitor, while the remainder shares the same path as the current loop. It is important to keep the current ripple small on the cable side because it will appear across the sense resistor as a voltage ripple, and the magnitude of the voltage ripple needs to be smaller than the ADC’s resolution specification for reading the sense resistor voltage. Current ripple can be further reduced by additional filters. An RC filter is a good design compromise because of its small input current and lower cost compared to an LC filter. Further smaller ripple currents can be achieved by using a two- or three-stage cascaded RC filter.

Using LTspice® simulations, we can compare the current ripple on the source cable side for three different input filter configurations, with a total resistance of 100 Ω in series in the input path, using the LT8618 (V_IN = 28 V, V_OUT = 5.5 V) and an 82 µH inductor. The current pulse is equivalent to what is seen by the input filter as the input current of the LT8618 regulator, and the output current is 10 mA.

A single stage RC filter with 100 Ω and 100 nF has over 60 µA peak-to-peak current ripple on the source cable side. If you add capacitors or cascade filter stages, the ripple current on the source cable side becomes smaller. Considering that buck regulators perform better with larger direct input capacitors, and that the BOM of the two-stage RC filter is smaller than the three-stage, while the current ripple on the source cable side is similar, we recommend using a two-stage filter , 50 Ω resistors and 47 nF capacitors are selected for each stage. The ripple current on the source cable side is about 30 μA, which corresponds to a ripple voltage of about 7.5 mV across the 250 Ω sense resistor, which is almost sufficient for an ADC with 8-bit resolution. To further reduce the cable side ripple current, larger capacitors can be used in the filter. For example, if the 47 nF capacitor is replaced with a 100 nF capacitor, the cable side ripple current can be reduced to only 7 µA, corresponding to a ripple voltage of 1.75 mV.

Figure 5. Current ripple on the supply side of the current loop.

In a typical current loop application, the customer would specify a current limit value (eg, 3.2 mA) during startup, but this limit can be exceeded for a specified short period of time. In a buck converter, a high inrush current is usually generated to charge the input capacitor. The function of the input filter is twofold: in addition to limiting the ripple current on the source side of the cable, it also helps limit the inrush current at startup. Figure 6 shows the input voltage V_INVariation of input current with time during startup of a two-stage input filter at 24 V and a load current of 4 mA on the output side.

Figure 6. Startup current with input filter used to limit inrush current (from top: input voltage 20 V/div, output voltage 5 V/div, enabled, input current on cable side, 10 mA/div)

in conclusion

Current loops are widely used in industrial and automotive systems to collect sensor information and transmit it to control systems, sometimes over relatively long wires. Conversely, the loop transmits controller outputs and modulation commands to remote actuators and other devices. By improving the power supply in the current loop, especially replacing traditional linear regulators with high-efficiency step-down regulators, significant improvements in efficiency and performance can be achieved, as well as increased current capability and wider input range. High-efficiency, high-input voltage regulators in small packages with low minimum on-times enable compact overall solutions that are comparable in size and robustness to LDO regulator solutions. This article describes how to use the LT8618 in a 4 mA to 20 mA current loop transmitter to meet stringent industrial requirements.