Control circuit apparatus and power supply circuit control method

A microcomputer (1) outputs a command to a control IC (2) to operate first and second loads (6, 9). The control IC (2) sets first and second DC/DC converters (4, 7) on-state to operate the first and second loads (6, 9). The first DC/DC converter (4) converts a voltage of power supplied from a power supply from 8.4V to 1.5V, and supplies it to a constant current circuit (5). The constant current circuit (5) calculates a current value of a load current to be consumed in the first load (6) and the second load (9) to which power is supplied via the second DC/DC converter (7), and compares it with an electrical current value at which the utmost conversion efficiency is achievable in the first DC/DC converter (4). If the load current is smaller than that, the constant current circuit (5) supplies a surplus power to a charging battery (8), and if the load current is greater than that, it receives a deficient power from the charging battery (8).