Nikkei Electronics Asia -- April 2007
Insights
Current Sensing in Automotive Applications

Automotive applications for current sensing include controlling current through solenoids and injectors, for example diesel fuel injection where a relatively high 48V or more is used to quickly ramp the current in the inductive injector up to 20A. Once the 20A is reached, current sensing provides feedback to control circuitry to keep the injector at 20A.

Current sensing often provides valuable performance enhancements or features. A power window system readily demonstrates the advantages brought by current sensing. Because motor torque is directly proportional to current, the motor can be shut down in excessive torque situations - such as when a human limb is caught in the window, or the mechanical system jams.

Current Sensing Methods
Current can be sensed on either the low side or high side of load or supply. The common-mode voltage is the voltage where the shunt is (as opposed to the differential voltage across the shunt), which in low side sensing is zero volts. Low side sensing is the simplest with very basic amplifier circuits. Difficulties with low side sensing include the facts that it disturbs the ground side of a system, may require extra wiring, and usually doesn't facilitate diagnostics.

The high-side shunt amplifier in Fig 1 senses a very low differential voltage (typically 100mV or less) riding on a much larger supply voltage, typically 13.8V in an automobile. However, if this is the unconditioned battery line, it can be subject to transients and reversals from 13.5V (a battery installed backwards by accident) up to a transient of 72V (such as load dumps or inductive kick-back).

Consider that the amplifier is usually powered with a single supply of 5 to 12V. This requires an amplifier whose input pins can be connected to common-mode potentials, well beyond the limits of its power supply rails.

Older shunt-sensing circuits were based on a difference amplifier, an operational amplifier (op amp) with four resistors around it to set gain and provide differential inputs. These resistors enable an op amp to accept common-mode voltages beyond its supply rails. However, there are tradeoffs such as: a) the circuit must be either configured as an attenuator and be able to recover the gain in a subsequent op amp stage. The gain of the op amp multiplies offsets and drift of the first amplifier degrading overall performance; and b) using a high common-mode voltage difference amplifier that includes additions to the resistor network enables it to accept high common-mode voltages while still providing unity gain. The tradeoff of the high common-mode difference amplifier is that the noise gain of the op amp is proportional to the common-mode attenuation. The packaged difference amplifier uses an internal 20:1 common-mode attenuation. Consequently, amplifier offset, drift, and noise are all 20 times worse than the op amp itself. Additionally, the large input resistors contribute significant noise.

Current shunt monitors overcome the limitations of resistor-based difference amplifiers. These are high common-mode voltage difference amplifiers targeted specifically at shunt current sensing. The primary difference in current shunt monitors when compared to difference amplifiers is that the common-mode voltage capability usually extends only to positive voltages. Some current shunt monitors allow common-mode to ground, which may cause significant ramifications. It also allows some negative voltages. Current shunt monitors are designed at the outset to operate on single supply voltages, usually down to as low as 2.7V. This depicts a current output type alongside a voltage output type. The current output shunt monitors typically have low quiescent current and require an external output resistor. This also enables the end user to program the gain. Voltage output devices are offered in fixed gains and require no additional components.

Technical Requirements
Automotive current sensing falls into two broad groups. It can be connected either directly to the battery or behind protective circuitry that limits transient excursions.

These considerations affect the requirements for common-mode voltage ratings on the current shunt monitor. While the automotive 12V electrical system operates at up to 14.4V, devices on the battery bus can see transients of up to 75V, and possibly battery reversals.

Another common-mode consideration is when the supply line has the shunt on and is shorted to ground (this is a common-mode voltage of zero). As the current must be measured while flowing, the amplifier needs to be functional at zero common-mode volts.

Lastly, take the case of pulse width modulation (PWM) solenoid drivers. The top of the solenoid is at battery voltage when turned on. When the switch turns off, this voltage will flyback to a negative level of a diode drop. This requires that the current shunt monitor be functional at common-mode voltages at up to -2V.

Current Comparison
In many applications current needs to be compared to some set point. Normally, implementing this requires a current shunt monitor, a comparator, and a reference voltage. Furthermore, the ideal comparator output is an output readily compatible with most customary logic circuits. An example of current comparison is shown in Fig 2. The current is compared to a simple single trip point using a TI INA200 series current shunt monitor and comparator. R1 and R2 form a voltage divider to set the trip point based on the built-in 0.6V trip point of the INA200 series comparator.

In some systems, more than one current limit level is required. For instance, a system may use a lower current limit as an indicator of impending difficulty, while the upper limit is used to shut the system down. The INA206-INA208 series of current shunt monitors include two comparators useful for implementing this function. Some systems may specify a delayed output on the lower limit to avoid unnecessary triggering on transient excursions beyond the lower limit.

by Jerry Steele,
Strategic Development Engineer,
Texas Instruments

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