TPS63802: TPS63802 TEC Application PWM frequency

Part Number: TPS63802

Dear TI:

Custom use tps63802 as TEC application, but i have some confused, can you some advise,thanks?

1,  Does  Vctrl must be PWM output or DAC will be ok?  if must be PWM, what's the requirement? frequency or something else..

2. Custom use PWM (Freq=2MHZ,Duty=15-20%) ,but no output voltage change ,
thanks