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Design News, February 2013

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s e n s o r s / m ac h i n e v i s i o n figure 2: allegro integrated flip-chip technology provides attractive new solutions for current sensing. nected in parallel (to reduce power loss), because this can cause parasitic ground-loops. The disadvantage of high-side sensing is that depending on the high-side voltage, the amplifier circuitry must be able to operate with high common-mode input voltage signals, making the design more complex and the solution more expensive. Low-side sensing relaxes the commonmode input requirements for the amplifier circuitry, but it is also more susceptible to disturbances in system ground potential, and it is unable to detect short-circuit conditions from supply to ground potential. In sense resistor implementations, measurement accuracy is largely limited by the temperature coefficient, TC, of the sense resistor and the input offset error, VOSI, of the amplifier. A smaller value sense-resistor usually results in degraded accuracy performance since the amplifier's input offset error now constitutes a larger percentage of the applied signal at the amplifier input. The use of a larger value sense resistor, while beneficial for output accuracy, results in higher power dissipation. As a result, the sense resistor value for a design is usually chosen based on a design trade-off between sensing accuracy and power dissipation. Consider that a typical current sense resistor value for low-side applications Sense resistor (20 mΩ) and amplifier Package aCS710 aCS711eX To-220 and soT-23 soicw-15 QFw 170 mm2 + 8 mm2 170 mm2 25 mm2 Total area 178 mm2 1.6X smaller 20X smaller Power Dissipation 1&w + 0.25nw 1w 0.6 w 18 W 18X less power 30X less power Isolation none 277 vac <100 vac Table 1: Competitive comparison for 30a current sense applications. is on the order of 20 mΩ. For a 30A continuous current-sense application, the power dissipation from resistive losses would be: PD = I2R = (30)2 × 0.02 = 18W This power, dissipated as heat, requires the use of often larger and more expensive, high wattage resistors that employ special thermal technology such as heat-sink mounting and exposed metallic pads for heat dissipation. This power loss also makes the solution energy-inefficient, an increasingly important consideration in portable electronics and ecofriendly design. Improved Current Sensing With Integrated Hall Technology Another drawback of sense resistor techniques is they often lead to a volume penalty in the application PCB because thermal considerations have to be taken into account when so much energy is being dissipated as heat. Figure 1 compares the minimum volumes and PCB footprints occupied by a typical sense resistor (TO-220) and op-amp (SOT-23) solution, with PCB consumption by Integrated Hall-effect based current sense techniques (QFN and SOIC). Allegro's integrated Hall-effect current sensor ICs use patented flip-chip technology, illustrated in figure 2, to integrate a high-accuracy, currentcarrying conductor into an area- and thermally efficient, monolithic package. Current flows in and out of the package through the primary conductor loop. This current generates a magnetic signal that is sensed by the Hall transducer, shown as the red square in figure 2, and is converted T10 Tre nd waT c h : sens o r s / m a c hi n e v i s i o n / a s u p p l e me nT To de s ign ne ws f eb r uary 2013 [www.designnews .com]

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