ZHCSMT3A november 2020 – march 2023 ALM2403-Q1
PRODUCTION DATA
The corner frequency of the 2nd-order MFB filter is set to approximately twenty times less than the PWM frequency. The corner frequency defined at –3 dB is shown in Equation 1.
The 2nd-order MFB active filter uses an inverted input topology and the op amp gain is determined by the ratios of resistors R2 and R1:
The gain settings are based on the output drive requirements and PWM signal amplitude. With different gain settings, the filter characteristics, such as rolloff, can change. The design must be fine-tuned to meet optimal performance needs.
The quality (Q) factor of the low-pass filter is configured with Q = 1. The purpose of designing for this Q factor is to minimize attenuation around the corner frequency of 10 kHz, thus extending the pass-band gain. The Q factor of the 2nd-order MFB filter is given by Equation 3: