DB2 SQL SELECT top 1 group with join

I'm connecting to a DB2 database and want to use the Max(PO180M1.POORDT) or the TOP 1 grouping by the PO180M2.PHPPN (PARTNO). How would I do this? I think I keep getting close but not getting it to work.

SELECT PO180M2.PHPPN AS PartNo, (PO180M2.PHVNPD || ' ' || PO180M2.PHVNP2) AS PartDesc, 
       PO180M1.POORDT AS OrderDate, PO180M2.PHUNCT AS UnitCost
FROM PO180M1, PO180M2 
WHERE PO180M1.POORNO = PO180M2.PHORNO 
ORDER BY PartNo, OrderDate DESC