DRV8305-Q1: DRV83053QPHPQ1 SOx voltage didn't steady(follow MOS output PWM frequency)

Hi TI

When I use DRV83053QPHPQ1 in my design, MCU control DRV8305 through 20KHz PWM，I found SO2 output didn't steady. 

You can see my waveform, yellow wire is SO2 output, blue wire is MOS output. you can see SO2 follow MOS output frequency.

In my design, I need a steady feedback SO2 voltage, how to fix this?