DRV8703-Q1: DRV8703QRHBRQ1 PWM control issue

Tool/software:

Hello , 

           Now ,we are use  DRV8703QRHBRQ1 to control motor , use PH/EN mode (Mode=0), and the PWM frequency is 20KHz. but we have one issue , input PWM signal from IN2/PH  but find the output PWM signal fixed is 20% , when we increase the PWM duty from 20% to 80%, and the output PWM duty always is 20% and frequency change to 40Khz， what is the reason and how to fixed it .

Thank you,

Alex