Use fast, accurate overcurrent detectors to diagnose automotive safety systems

Figure 1 shows a typical overcurrent circuit, which includes a discrete operational amplifier and a discrete comparator.

Figure 1: Overcurrent detection using discrete op amps and comparators

Sources that determine the accuracy and response time of the system include:

• Shunt resistor (RS) tolerance and drift.
  

• Amplifier circuit gain errors (RI and RF).
  

• Error in the voltage divider (R1 and R2) between the amplifier and the comparator.
  

• Comparator reference (R3 and R4) input error.
  

• The response time of the amplifier circuit.
  

• Comparator response time.

In the worst case, errors caused by the current shunt, gain, and voltage divider between the amplifier and the comparator will result in current measurement errors. Depending on this level of error, you must build margin into your design to ensure that it does not exceed the desired operating parameters.

The total fault condition response time includes not only the amplifier and comparator response times, but you must also consider the microcontroller (MCU) cycle time and the protection circuit turn-on and turn-off times. The total response time must be shorter than the time it takes for the system to enter a safe operating state. Typically, the MCU cycle time and protection circuits are fixed; therefore, you must adjust the amplifier and comparator response times to meet system requirements. TI offers solutions for amplifiers and comparators with various response times.

If you want to improve the current sensing accuracy, you can choose to use high-precision, low-drift resistors as shown in the circuit in Figure 1; however, as the accuracy and drift of the external components increase, the cost also increases. Another approach is to use a current sense amplifier, such as the TI INA185. Current sense amplifiers integrate a precision matched resistor gain network to cost-effectively reduce gain error and drift. For the INA185, the gain error is ±0.25% and the drift is 8 ppm/°C, or ±0.33% over temperature.

此外，您还可以通过使用性能更佳的电阻器来改善参考误差。使用带有集成精密基准电压的比较器（如TI TLV4021）也可在温度误差为±0.04%的情况内显著改善参考误差。图2所示为同时使用INA185和TLV4021用于过流检测电路的电路。

Figure 2: Overcurrent detection using a precision current sense amplifier and a precision comparator

This makes the voltage divider error the dominant error source. You can eliminate this error source by using a current sense amplifier such as the TI INA301. This amplifier integrates a comparator and a precision voltage reference, as shown in Figure 3.

Figure 3: INa301 functional block diagram

The INA301 has an on-chip precision current source that requires only one external resistor to set the threshold. In addition, the total response time of the alert output is less than 1μs.

Monitoring the current state of a system is a leading indicator of potential problems. Improving overcurrent detection accuracy can improve system power efficiency by minimizing allocation headroom. There are many overcurrent detection solutions that can be optimized based on the key issues of a specific application: cost, solution size, accuracy, or response time. You can trade off the lower cost of typical discrete components with the higher accuracy provided by current sense amplifiers and comparators with integrated references.