8.2.1.2 Detailed Design Procedure

Although the device is only able to measure current through a current-sensing resistor flowing in one direction, a second INA381 can be used to create a bidirectional monitor. With the input pins of a second device reversed across the same current-sensing resistor, the second device is now able to detect current flowing in the other direction relative to the first device; see Figure 50. The outputs of each device connect to an AND gate to detect if either of the limit threshold levels are exceeded. As shown in Table 7, the output of the AND gate is high if neither overcurrent limit thresholds are exceeded. A low output state of the AND gate indicates that the positive overcurrent limit or the negative overcurrent limit has been exceeded.

In this scenario, the maximum current expected through the shunt resistor is 20 A in either the forward or reverse direction. Maximum accuracy is desired; therefore, the shunt resistor is maximized by taking the maximum output swing divided by the smallest gain and divided by the maximum current. The design parameters used in Table 6 yield a shunt value of 12.3 mΩ. The closest standard 1% and 0.1% device is 12 mΩ, and this value is used by both INA381 devices.

Because corrective action must be taken when the current exceeds ±19 A, the comparators require a value of 4.56 V (19 A × 0.012 Ω × 20 V/V). In this instance, a voltage divider consisting of two 4.53-kΩ resistors (R1 and R3) and two 5-kΩ resistors (R2 and R4) off the 5-V rail supply a voltage close to this value. To be certain that both device alert functions can trigger a single GPIO pin on a microcontroller, both comparator outputs feed into an AND gate.