Current-Sense Amplifier Doubles as High-CM IA

Nov 1, 2008 12:00 PM
By Alfredo H. Saab, Applications Engineering Manager, and Ahmad Ayar, Member of Technical Staff, Maxim Integrated Products, Sunnyvale, Calif.

A current-sense amplifier senses the voltage developed by a precision sense resistor connected across its differential inputs. The resistor sits at a voltage level higher than that of other supplies used in the system. The output, a scaled-up, precision, single-ended replica of the differential input voltage, is referred to the system equipotential (ground).

For a given current value, the precision gain of the current-sense amplifier reduces the voltage burden imposed by the sense resistor on the line in which current is measured, because less drop in the sense resistor is required for the output voltage needed to measure it. Therefore, a current-sense amplifier complies with the basic description of a voltage instrumentation amplifier (IA), in that it is a precision-gain differential amplifier.

The main difference between IAs and current-sense amplifiers is that IAs generally operate in the four quadrants defined by an input-voltage axis and the orthogonal common-mode-voltage (CMV) axis (±input voltage, ±CMV). In contrast, standard current-sense amplifiers usually operate in one quadrant (+input voltage, +CMV), but also sometimes in two quadrants (±input voltage, +CMV). For current-sense amplifiers, the polarity of the measured current determines the sign of the input voltage. Also, CMV ranges are wider for current-sense amplifiers.

Current sensing is not the only application that benefits from an ability to amplify signals with precision and deliver them at levels separated by a large voltage difference. The standard application of Fig. 1 (a ground-referenced -48-V to +5-V power converter) shows the capability of today's current-sense amplifiers.

It is easy (in concept) to design a switching converter whose input source and output voltage are of opposite polarity. When it comes time for the engineering details, however, the choice of circuit topology is more difficult. For converters that operate with a standard positive-energy source, the reference levels for output voltage and regulator feedback voltage are the same: the negative side of the energy source. In this case the level is -48 V, as defined by the converter's positive-source topology.

This -48-V reference contradicts the intent of the application, which assumes that the low-voltage positive output must be regulated with respect to the common (ground) point. Isolated topologies (flyback, forward) that operate with different reference points for the input voltage and regulated output are preferred for this type of application, despite their higher costs and more complicated circuitry.

For the simpler solution of Fig. 1, a standard switching converter operates in a nonisolated topology. In place of the conventional transformer/optocoupler design that isolates, separates and shifts the output-sense level to the converter's regulation point, it employs for that purpose a current-sense amplifier (MAX4080F). The converter IC in this example is a MAX668.

The MAX668 stepup converter has a regulation setpoint of 1.25 V at the FB terminal, and the MAX4080F has a gain of 5 between the voltage at its differential input and the voltage between its OUT and GND terminals. The differential-input voltage necessary to produce 1.25 V at the converter's FB terminal is 0.250 V. When the system is in regulation, a voltage divider connected between the +5-V output and system ground (common) produces 0.250 V as required at the MAX4080F differential input.

Fig. 2 illustrates circuit regulation at a constant 1-A load for both the 5-V and 3.3-V versions, and Fig. 3 shows the efficiency versus load current for both versions. The maximum allowed voltage difference between the MAX4080F inputs and its GND terminal is 75 V.

The Origins of the -48 V

To protect telephone wires from electrolytic corrosion, the first telephone-system exchanges employed a “central-battery” power supply whose polarity was negative with respect to ground (earth). And, to ensure good low-noise contacts in the relays used in those systems, the supply voltage (-48 V) was made higher than that of most other battery-powered systems.

Since the early1960s, however, electronics systems have evolved in another direction. Driven by the dominance of npn bipolar transistors as the reigning active devices, almost all power supplies for today's analog and digital systems generate voltages that are positive with respect to the reference equipotential (ground).

Because the bulk of today's telecom power is distributed and used much as it was in the early days, the main power source is still -48 V with a large battery backup. Telecom systems, on the other hand, are now totally electronic and require low-voltage positive power-supply lines. Thus, the generation of low-voltage positive power from -48 systems is a common requirement.

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