DRV10974: how to control PWM to right speed?

Part Number:DRV10974

Dear Engineer:

with DRV10974 used ,do we need to change the pwm duty cycle when motor start up?

for example:

we need cw and motor run at  special speed(50% pwm duty cycle) , when motor start up ,should we first provide pwm duty cycle under 15% and then  incease the duty cycle step by step to 50%;

or the only thing need to be done is set the FR and set the pwm duty cycle to 50%?

Our hardware shows that ,when we just set the FR and pwm duty cycle to 50%,the motor speed up to a high speed ,and then down to the right speed we need,can you please help us figure out the cause?

Do you have a good way to solve the high speed problem?