Fan keep running when ac is on, and coolant temperature always low

What is your question?

When ac is off, my cooling fan works fine, sometime on and sometime off, and coolant temperature is optimal around 90 - 100 C. But when I turned AC on, the fan will keep running, causing my coolant temperature to drop low to around 70 -75 C and always stay there and never go above that no matter how. When I turned the AC off, then after like 1 min, the fan will also off, and coolant temperature is optimal again. Is it normal? What may be the cause? Thank you My car has 41000 miles. My car has an automatic transmission.

What makes this problem better or worse?

Turned ac off will gets thing to normal