Protection circuit
Response time
Scope photo
In most modern cars, an engine control unit (ECU) or engine control module (ECM) monitors various sensors throughout the engine, controlling the amount of fuel injected, ignition timing, variable cam timing, and other peripherals. If an error condition causes the load current to exceed a preset limit, the ECU must detect it and shut down the related circuit to protect electronic components. The circuit in Figure 1 can detect an overcurrent condition and send an alarm signal to the ECU within 100 ns.
The circuit employs an AD8214 high-voltage threshold detector. The chip’s 2.4-V series regulator, referenced to the supply voltage, allows the circuit to be powered by the car’s battery with no degradation of performance as the battery voltage varies. Fast, high common-mode voltage threshold detectors such as this with a current output operate as extremely fast comparators. The circuit detects overcurrent conditions on the high side of the control loop and quickly shuts down the control loop, preventing damage caused by excessive current flow to the load through the shunt resistor.
The threshold detector’s output current is less than 100 nA when In– exceeds In+ and is 1 mA when In+ exceeds In–. The input-to-output response time is typically less than 100 ns, even with a small overdrive voltage (Fig. 2) (In+ – In–). Hysteresis (10 mV typical) minimizes the effects of system noise, preventing false triggering.
In operation, a voltage drop is created across a small series resistor, Rshunt, which carries the main supply current to the load. This voltage drop drives the inverting input of the comparator negative with respect to the positive supply. R1 and R2 form a voltage divider across the regulator. The comparator’s other input connects to this divider, so the comparator trips as the voltage across the shunt resistor crosses the selected threshold.
The circuit’s output current, 100 nA or 1 mA, drives a ground referenced resistor to develop logic levels, as determined by the value of the load resistor RL. With RL = 5 kΩ, the digital output, D_OUT, can be connected to the ECU to signal an alarm condition at the load.
As noted, a 10-mV hysteresis is activated when In– is driven negative with respect to In+ and the comparator output switches from off to on. Consequently, to restore the output to zero, the input polarity must be reversed by 10 mV beyond the original threshold.
The comparator’s threshold voltage is set by R1 and R2. For a desired maximum current of 10 A, and Rshunt = 0.005 Ω, the threshold input voltage is VTH = 10 A × 0.005 Ω = 50 mV. The total voltage across R1 and R2 is always 2.4 V. The values for these resistors are chosen based on the desired threshold voltage using the equation:
VTH = 2.4 V\\[R1/(R1+R2)\\]
To support a 10-A trip point, the values of R1 and R2 are 1.6 kΩ and 75 kΩ, respectively.
If the input signal changes slowly enough to affect the propagation delay, the error that accumulates at the input while waiting for the output to respond will typically be less than 15 mV for ramp rates less than 100 mV/µs.
The circuit’s ability to handle a very large load current makes it useful for many automotive applications. Because the AD8214 is powered directly from the battery, the shunt resistor must be on the high side. The device will monitor the load current as long as the battery voltage is between 5 V and 65 V.
The hysteresis is also an asset in the noisy environment of automotive applications (Fig. 3). When the threshold is crossed, the output switches from low to high very quickly. But when the input voltage recovers, the output only switches back (from high to low) at the new threshold of 10 mV more.