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How to Power Smart Multivariable Sensor Transmitters

| Author / Editor: Timothy Hegarty * / Matthias Back

Loop-Powered Buck Converter for Sensor Transmitters

To increase the available supply current for an advanced, loop-powered sensor transmitter in excess of the 3.6 mA maximally available from the 4- to 20-mA loop, a DC/DC converter with high efficiency provides an inherent current-multiplication feature not possible with a traditional LDO regulator. A high-efficiency synchronous buck converter, such as the LM5165 from TI, optimized for a load-current range of 1 mA to 25 mA is a good choice. [4] Figure 4 shows the converter schematic with a switching frequency of approximately 220 kHz at nominal 24-VDC input. The power solution is easy to use, requiring no loop compensation components, and the 3.3-V fixed-output version requires only an inductor and two capacitors for operation.

Various features integrated for reduced size and enhanced reliability include a cycle-by-cycle current limit, an internally-fixed or externally-adjustable output softstart (SS), precision enable with customizable hysteresis for programmable line undervoltage lockout (UVLO), and an open-drain PGOOD indicator for sequencing and fault reporting.

Electromagnetic Compatibility Performance

Electromagnetic compatibility (EMC) is a key consideration in any electronics product development and critical for systems integration. Table 2 lists several important EMC standards and suggested test levels for sensor applications.

The input filter in Figure 4 includes Schottky diodes for input reverse-polarity protection and a transient-voltage suppressor (TVS) diode for surge protection. A resistor in series with the converter input, designated RIN, is typical for circuits with a 4- to 20-mA current loop. It not only provides damped EMI filtering, input-ripple attenuation and inrush protection, but also contributes to small-signal stability of the current loop. A loop current-sense resistor of 40 W is shown explicitly in the schematic even though it is typically integrated into the loop driver (Figure 1).

Conformance to EMI Regulatory Specifications

System-level conformance to EMI regulatory specifications is an increasingly-important power-solution benchmark and a crucial milestone in a product’s design cycle. For high-density sensor designs in particular, there is little space available for EMI filtering. Moreover, the imperative is that the switching power converter should not affect the sensor’s functionality.

The LM5165 buck converter incorporates two features to minimize its EMI signature. First, an integrated active-slew-rate control of the switch-node (SW) voltage transition lowers both conducted and radiated EMI. As shown in Figure 5, the current-source gate driver discharges the high-side MOSFET’s non-linear gate-drain capacitance, CGD, so that the SW-voltage overshoot and ringing are eliminated. The capacitance of CGD increases as the VDS voltage decreases, corresponding to the increases in SW voltage. Also, the current-source gate driver tunes the slew-rate profile of the SW voltage as it swings from GND to VIN during the turn-on transition of Q1. The result is a low-noise turn-on transition for Q1, eliminating SW-voltage overshoot and ringing.

Second, the LM5165’s PFM control mode uses a boundary conduction mode of switching to allow a lossless and soft turn-on transition for the high-side MOSFET. The turn-on of the high-side MOSFET occurs at zero inductor current, thus eliminating reverse recovery losses related to conduction of the low-side MOSFET’s body diode.

The cumulative benefits of these switching techniques are increased reliability and robustness owing to lower voltage and current stress, and increased margin for input-voltage transients. There is also more tolerance to non-optimized board layouts, and easier EMI filtering, particularly in the more challenging high-frequency band above 30 MHz. [7] Based on the CISPR-25 class-5 EMI test setup and limits, Figure 6 shows the conducted emissions plot from 30 MHz to 108 MHz for the converter shown in Figure 4.

DC/DC Converters with Distinctive Requirements

DC/DC converters for powering smart sensor-transmitter nodes in noisy industrial environments have distinctive requirements such as high efficiency, wide VIN, low IQ, small form factor, robust EMC performance, and low noise. This article presented a transmitter design with BLE connectivity for a multivariable sensor. The LM5165 wide-VIN synchronous buck converter offers a compact power solution with low EMI that can reduce time-to-market and total solution cost.

References:

[1] “Field Transmitter with BLE Connectivity Powered from 4 to 20-mA Current Loop”; TI-Referenzdesign TIDA-00666, [2] “Dual Sensor Measurement Using Single Current-Loop with FSK Modulation”; TI-Referenzdesign TIDA-00483, [3] “Hall Sensor Proximity Sensor w/Fixed Threshold And Temperature Sensing Over IO-Link”; TI-Referenzdesign TIDA-00340, [4] Timothy Hegarty: “Low-IQ synchronous buck converter enables intelligent field-sensor applications”; Texas Instruments Analog Applications Journal (SLYT671), 2. Quartal 2016, [5] “Bluetooth low energy software stack,” Texas Instruments full-featured, royalty-free Bluetooth 4.2 certified stack (BLE-STACK), Juni 2016, [6] Timothy Hegarty: “Valuing wide VIN, low EMI synchronous buck circuits for cost-driven, demanding applications”; TI Whitepaper SLYY104, Oktober 2016. [7] Timothy Hegarty, “Reduce buck-converter EMI and voltage stress by minimizing inductive parasitics,” Texas Instruments Analog Applications Journal (SLYT682), 3Q 2016 Related Web

* Timothy Hegarty is Systems Engineer, Non-Isolated Power Solutions, at Texas Instruments.

* This article was first published in Elektronikpraxis

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