Answer Page

(V_DD)2/(2pi²R_L)=
                        =(5²)/(2pi²8)
                          =25/158
                          =158mW
See equation in section 4.2 of AN 1192, page 2.

(4)(V_DD)²/(2pi²R_L)=
                              =(4)(5²)/(2pi²8)
                              =100/158
                              =633mW
See equation in section 4.2 of AN 1192, page 3.

The maximum power dissipation value is required to determine the proper heat sink for an application. See LM3886 datasheet, page 14.

120mm²
See graphs in LM4890 datasheet, pages 13 and 14.