BQ500211A: Chip can't generate a serial PWM signal

Part Number:BQ500211A

hi:

     I design the power transfer circult by the "Typical Low-Cost Application Diagram "circult on the page 20 of the datasheet.But the chip can not work normally.

The pin12,pini13 (DPWM-A,DPWM-B) can't generate a serial wave. It generates a short time PWM wave around every 6 seconds.I can't find the solution of the problem.

regards