some questions about ADF4159

Hi all

when i use ADF4159 to generate a chirp signal.

i want to control the time of down rate of the chip . just lower the down rate of chip.

for example , i create a chirp signal , the ramp up time  is 50us . and i want to change the ramp down time to 6us .but now i use sawtooth wave, the ramp down time is so small . and i can not control the ramp down time.

I read the datasheet and try some methods. but i can not get it . So could you help me,

Thanks .