SLVSGT3 December 2022 DRV8317
PRODUCTION DATA
The SOx pins are typically sampled by an analog-to-digital converter in the MCU to calculate the phase current. Phase current information is used for closed-loop control such as Field-Oriented Control (FOC).
An example calculation for phase current is shown in Equation 3.
For example, in a system with VREF = 3-V, CSA gain = 0.5 V/A, and a SOx voltage of 1.2-V, phase current (IOUTx) = -0.6A.
Sometimes high frequency noise can appear at the SOx signals based on voltage ripple at VREF, added inductance at the SOx traces, or routing of SOx traces near high frequency components. It is recommended to add a low-pass RC filter close to the MCU with cut-off frequency at least 10 times the PWM switching frequency for trapezoidal commutation and 100 times the PWM switching frequency for sinusoidal commutation to filter high frequency noise. A recommended RC filter is 330-ohms, 22-pF to add minimal parallel capacitance to the ADC and current mirroring circuitry without increasing the settling time of the CSA output.
The cutoff frequency for the low-pass RC filter is in Equation 4.