Use of current sense measurements in automotive systems

Contemporary automotive electrical system design is experiencing one of the most dynamic periods of change seen in history. Everything from revolutionary dual use motor/generator hybrid electric propulsion and "fly-by-wire" electric actuators, to service-life and efficiency-enhancing intelligent accessories like belt-less pumps and LED illumination, is being rapidly integrated into new vehicles. Growing customer expectations of automated on-board diagnostics and predictive maintenance features also drive new paradigms in the body and engine management system design. In many of these areas of system redesign, a key piece of informational feedback is the electrical current being used by particular loads. The current measurements are used to perform state-of-health analyses, faultprotection, and provide control-law support. The fundamental shift is that traditional "open-loop" systems of the past are being supplanted by smart and efficient "closed-loop" designs.

BASIC CURRENT SENSING TOPOLOGIES
While non-contact current measurement is possible, it is generally relegated to high-cost instrumentation or high-value power-unit products where the cost and complexity are justified. In the automotive area, low cost is key and in this respect the senseresistor technique is the most appropriate. By inserting a small known sense-resistance (on the order of just a few milliohms) in series with the load, and measuring the small resultant voltage-drop across that resistance while the load is powered, one can accurately deduce the current flow.

There are fundamentally six different topologies for the series connection of a switch, a load, and a sense-resistor, as shown in Figures 1(a) through 1(f). They can be classified by the location of the switch as either high side or low side with respect to the load; and by the resistor location as low side, "flying", or high side with respect to the supply rails. Each of these scenarios has the possibility of being an optimal solution for some particular application. Another consideration is that of fault scenarios, which may vary according to the load properties. As a rule of thumb, one would typically assume that the most probable fault is a connection to frame (electrical ground), either by a wrench touching a live exposed terminal or a chafed wire that contacts grounded metal work, in which case low-side sensing would be intrinsically deficient. In most applications, the configuration of Figure 1(c) is the topology of choice, as it allows centralizing the switch and monitor functions while maintaining a low wire count.
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MODERN LOADS & SMART SWITCHES
Since the introduction of power MOSFET devices, designers have seen them as potential replacements for relays. Contemporary N-MOSFET switches provide on-resistance characteristics in the single-digit milliohm range, permitting use of standard surface-mount techniques without bulky heat-sinking structures. Inexpensive integrated-circuit (IC) solutions have been developed that provide self-contained voltage boosting gate-drive features. These same circuits employ fast faultprotection mechanisms so that the MOSFET is never stressed to the point of being at risk of failing. One such "smart switch" control IC is the LT1910 from Linear Technology, which utilizes a lowresistance high-side current-sense resistor (like in Figure 1(c)) to detect circuit overload and shut down the active MOSFET before damage can occur. On detecting an overload, the IC sets a warning flag and periodically retries activating the load until such time as the fault clears. While only binary in nature, this is a good example of current sensing used to form a robust "closed-loop" electronic relay solution as shown in Figure 2.

REAL-TIME CURRENT MONITORING
Aside from the smart switch protection that current sensing offers, amplifying and translating the sense-resistor signal allows digitizing the information as an "analog" feedback signal to a control-loop. Many loads exhibit operational properties that current monitoring can reveal in real time. Motors for example, draw current that is proportional to the delivered torque, so it is possible to deduce trends in bearing drag and detect various actuator conditions without using other sensors. Other loads, such as illumination, are frequently driven in parallel from a common power feed, so it becomes a matter of precision to identify if some portion of the load has failed open at end-of-life.

One particularly simple IC solution for this is known as a current-sense amplifier, the LTC6101 from Linear Technology being an example that is optimized for unidirectional high-side automotive sensing. Figure 3 shows a typical circuit example using the LTC6101 to interface a generic current measurement to an analog-to-digital converter (ADC) input. This circuit also shows added components D1 & R3 that provide reverse-supply transient protection. Table 1 shows a sampling of available sense amplifiers and their primary characteristics.

CONSIDERATIONS WITH PULSE-MODULATED LOADS
With loads that are duty-cycle modulated to produce variable performance levels by using highfrequency pulse-width-modulation (PWM) techniques, there are additional factors to consider in designing current monitor circuitry. The primary one is that responsetime needs to be fast enough to provide response to fault conditions within the on-time portion of the waveform, and another is that the switching activity should not significantly disturb the current readout fidelity. Usually the Figure 1(c) configuration again provides the best results, since the circuit impedances are low and commonmode issues are minimal. In situations where the average load current (the "dc-component") is desired, post filtering in either the analog or digital-signal-processing (DSP) domains can be employed to eliminate the PWM-related frequency components. The averaged supply current values are predictably related to the load current and provide a good indication of the subjective effect, be it lamp intensity or actuator force.

MONITORING H-BRIDGE DRIVER CURRENT
An H-bridge driver can be thought of as a pair of half-bridge sections operating with complementary signals so as to produce a differential bidirectional output. In turn each half-bridge may be considered an extension of the unidirectional Figure 1(c) configuration by the addition of the low-side switch in parallel with the load. Figure 4 shows such a configuration using an LTC6103 that generates a differential output suitable for driving an ADC directly. A circuit like this would be appropriate for motors in window lifts, climate-control mechanisms, and wherever reversible motions are performed.

Notice that for load connection faults to ground, the low-side MOSFETs are not subject to overstress, therefore monitoring each half-bridge on the high side provides all the needed information. Load current can be determined by taking the difference in the unidirectional current readings of the two halfbridges. Note that with signmagnitude control, where one of the high-side switches is 100% on, no duty-cycle correction is required to accurately measure the load current.

CONCLUSION
Electronically driven functions are proliferating in contemporary vehicle development. Along with the need for robust, yet costeffective control designs comes the added value of diagnostic capabilities provided by closedloop monitoring of load currents in the system. High-side current sensing provides the most practical means of implementing the monitor function whether the driver is single-ended or H-bridge. The high-side monitor function is easily realized with the LTC6100 family of current sense amplifiers, a growing series of ICs targeting these applications with compactness and high performance.