To obtain a power source switching unit that is capable of cost and size reduction,
and also supplying electric power reliably even when external power supply is intercepted,
and to obtain a computer that is capable of avoiding shutdown that results from
the interception of the external power supply. The power source switching unit
is equipped on a power supply line with an AC adapter detection circuit and outputting
a #AC-ADAP signal, and a temporary power supply circuit for temporarily supplying
electric power to a DC-DC converter when the power supply is intercepted. The switching
unit is further equipped with a battery switching circuit for switching the states
of FET1, FET2, FET3, and FET4 so that main and second
batteries are both connected to the DC-DC converter in response to a change in
the #AC-ADAP signal when the AC adapter is disconnected from the switching unit.
