You might need to come down from 85 volt to 5. Current might be 100mA. As much as 80 volt needs to be dropped. Any linear solution will produce 8 Watt. For any small sized box that is much, too much.
How much power discipation can You allow? All linear 5 volt voltage stabs also produce heat.
A switched regulator looks like the best. Maybe You need a custome designed regulator if no rare and special one can be found.
Is there anything You can do to reduce the main voltage? 85 volt, where is that used?