The INA745x is a precise, low-drift, digital power monitor that provides accurate measurements over the entire specified ambient temperature range of –40°C to 125°C. The integrated current-sensing resistor is internally compensated to provide measurement stability over temperature, while simplifying printed circuit board (PCB) layout and size constraints.

Access to the on-chip current-sensing resistor is provided by the IS+ and IS- pins. This resistor features internal sense connections that are brought out on the SH+ and SH- pins. Access to the digital power monitor is provided by the IN+ and IN- pins. When the shunt sense connections are connected to the digital power monitor inputs, the sensed voltage is calibrated and temperature compensated to achieve a high level of accuracy. The construction of this resistor does not allow the device to be used as a stand-alone component for accurate current measurement. The INA745x is system-calibrated to verify that the current-sensing resistor and digital power monitor are both precisely matched to one another.

The nominal pin-to-pin resistance from IS+ to IS- is approximately 1mΩ, while the internal resistance seen by the SH+ and SH- pins is nominally 800μΩ. The power dissipation requirements of the system and package are based on the total package resistance between the IS+ and IS– pins.

Figure 6-1 IS+ to IS– Package Resistance vs Temperature

The internal compensation of the INA745x corrects for pin-to-pin resistance increases with temperature, achieving low drift over the ambient temperature range.

The INA745x is most accurate when measuring currents around 15A to 20A. As currents increase, the error in the current measurement also increases. Figure 6-2 shows how the gain error of the INA745x varies with the shunt current for all device options.

Figure 6-2 Gain Error vs Shunt Current

The shape of this curve varies based on the PCB design. Designs with better thermal performance typically flatten this curve.

The temperature coefficient of the shunt is compensated by sampling the junction temperature and internally applying a correction factor based on this temperature to the reported current measurements. During transient currents, the shunt heats faster than the temperature sensor. This temporary difference in temperatures results in higher values in the reported current until the temperature stabilizes. Figure 6-3 shows the sampled output response during a current step from 0A to 22.5A

Figure 6-3 Device Current Transient Overshoot

The transient overshoot is similar to what is observed in analog output current sense amplifiers. Understanding that this overshoot occurs when setting an overcurrent alert threshold to avoid false triggers is important.