DRV8843: Why is the PWM frequency not fixed?

Tool/software:

Hello TI expert team,

I'm using DRV8843 for stepper motor drive. I found out there's a saying in datasheet: "The current through the motor windings is regulated using a fixed-frequency PWM scheme."

In my test result, the frequency is changing, normally the frequency is 50kHz, but when the motor current increase, frequency can't stay at fixed status. Especially when current through motor reached around peak value, the frequency is about 24kHz. Here I've attached the waveform.

Thank you.

Sincerely,

Maiyue Zhang