If condition

[Hasupama]

Hi
I am solving continuity equation with a constant time step. But I need to use dynamic time stepping.

t = 0.0 #STARTING TIME
dt_min = 1e-6 # Minimum time step [s]
tMax = 20e-3 # FINISHING TIME [s]
Dt = Constant(1e-3) #INITIAL TIME STEP [s]
while t <= tMax:
solve (PDE)
If it does not converge use Dt=Dt/2 and solve again.
Dt can be reduced until until Dt=dt_min

Could you please explain how to implement above if condition until Dt=dt_min inside the while loop?
The error message I got is diverged_DTOL. I need to reduce the time step when I get this error