SQL & PL/SQL

When restricting data based on a parameter I usually use the following:

AND p.start_date = NVL (p_start_date, p.start_date)

so IF the parameter is null THEN I just join on itself, ELSE we filter on parameter.

New requirement involves greater than or equal to, so I tried this:

AND CASE
WHEN (p_start_date = NULL) THEN
p.start_date = p.start_date
ELSE
p.start_date >= p_start_date
END;

but this is, at a minimum, is syntactically wrong.

Any suggestions?

Thanks!