Recent advancements in integrated Hall-effect current sensor technology provide an alternative current sensing solution that reduces power loss, achieves most cost targets, and occupies a much smaller volume on the application printed circuit board.
Current sensing with a sense resistor and amplifier
Conventional current sensing techniques insert a sense resistor in series with the conductor carrying the current being measured. An amplifier is also required, so that when current flows through the resistor, the voltage developed can be used to measure the input current. The value (usually ranging from 1 to 100 mΩ) of the sense resistor depends on the maximum target current that is to be sensed. Smaller sense resistor values develop a lower signal-voltage when current is applied.
Figure 1: Application PCB volume comparison.
Figure 2: Allegro integrated flip-chip technology provides attractive new solutions for current sensing.
The resistor-amplifier sensing circuit is implemented as a shunt circuit, either on the low side (near ground potential) or on the high side (near supply potential) of the load that carries the applied current.
High-side current sensing allows the detection of short-circuit conditions to ground potential, and is largely immune to ground potential disturbances. Ground potential disturbances become a greater concern when multiple low-side current sense resistors are connected 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 common-mode input requirements for the amplifier circuitry, but it is also more susceptible to disturbances in system ground potential, and 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 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