9.2.1 Design Requirements

The INA220-Q1 measures the voltage across a current-sensing resistor ®_SHUNT) when current passes through the resistor. The device also measures the bus supply voltage, and calculates power when calibrated. This section goes through the steps to program the device for power measurements, and shows the register results in Table 8. The Conditions for the example circuit is: Maximum expected load current = 15 A, Nominal load current = 10 A, VCM = 12 V, R_SHUNT = 2 mΩ, V_SHUNT FSR = 40 mV (PGA = /1), and BRNG = 0 (VBUS range = 16 V).

9.2.2 Detailed Design Procedure

In this example, the 10-A load creates a differential voltage of 20 mV across a 2-mΩ shunt resistor. The voltage present at the IN– pin is equal to the common-mode voltage minus the differential drop across the resistor. The bus voltage for the INA220-Q1 is measured at the external VBUS input pin, which in this example is connected to the IN– pin to measure the voltage level delivered to the load. For this example, the voltage at the IN– pin is 11.98 V. For this particular range (40-mV full-scale), this small difference is not a significant deviation from the 12-V common-mode voltage. However, at larger full-scale ranges, this deviation can be much larger.

Note that the Bus Voltage register bits are not right-aligned. To compute the value of the Bus Voltage register contents using the LSB of 4 mV, the register must be shifted right by three bits. This shift puts the BD0 bit in the LSB position so that the contents can be multiplied by the 4-mV LSB value to compute the bus voltage measured by the device. The shifted value of the bus voltage register contents is now equal to BB3h, a decimal equivalent of 2995. This value of 2995 multiplied by the 4-mV LSB results in a value of 11.98 V.

The Calibration register (05h) is set to provide the device information about the current shunt resistor that was used to create the measured shunt voltage. By knowing the value of the shunt resistor, the device can then calculate the amount of current that created the measured shunt voltage drop. The first step when calculating the calibration value is setting the current LSB. The Calibration register value is based on a calculation that has its precision capability limited by the size of the register and the Current register LSB. The device can measure bidirectional current; thus, the MSB of the Current register is a sign bit that allows for the rest of the 15 bits to be used for the Current register value. For this example, the minimum current LSB would be 457.78 µA/bit assuming a maximum expected current of 15 A using Equation 2. For this example, a value of 1 mA/bit was chosen for the current LSB. Setting the current LSB to this value allows for sufficient precision while serving to simplify the math as well. Using Equation 1 results in a Calibration register value of 20480 or 5000h.

The Current register (04h) is internally calculated by multiplying the shunt voltage contents by the Calibration register and then dividing by 4096 using Equation 4. For this example, the shunt voltage of 2000 is multiplied by the Calibration register of 20480 and then divided by 4096 to yield a Current register value of 10000 (2710h).

The Power register (03h) is internally calculated by multiplying the Current register value of 10000 by the Bus Voltage register value of 2995 and then dividing by 5000 using Equation 5. For this example, the Power register contents are 5990 (1766h). Multiplying this result by the power LSB that is 20 times the 1 × 10^–3 current LSB, or 20 × 10^–3, results in a power calculation of 5990 × 20 mW/bit, which equals 119.8 W. This result matches what is expected for this register. A manual calculation for the power being delivered to the load would use 11.98 V
(12 VCM – 20 mV shunt drop) multiplied by the load current of 10 A to give a 119.8-W result.

Figure 30. Example Circuit Configuration