Patent ID: 7863833

Claim:
A control circuit which controls a switching operation of a switching transistor of a separately excited DC/DC converter, the control circuit comprising: a switching controller which controls the switching operation of the switching transistor; and a voltage comparator which compares output voltage of the separately excited DC/DC converter and a predetermined threshold voltage, to detect a short-circuit state; wherein after a predetermined start-up time has elapsed after beginning start-up of the separately excited DC/DC converter, when the voltage comparator detects the short-circuit state, the switching controller halts the switching operation of the switching transistor, and makes detection of the short-circuit state by the comparator non-operative before elapse of the start-up time, and wherein the switching controller comprises: a state machine which holds a control state of the switching transistor, the state machine comprising three modes: a start-up mode in which detection of a short-circuit state by the comparator is made non-operative, while executing a step-up operation by the separately excited DC/DC converter, a normal mode in which detection of a short-circuit state by the voltage comparator is performed, while executing the step-up operation by the separately excited DC/DC converter, and a halt mode in which the step-up operation by the separately excited DC/DC converter is halted, and a transition is made to the normal mode after the start-up time has elapsed after transiting to the start-up mode, a transition is made to the halt mode when the voltage comparator detects a short-circuit state when in the normal mode, and a transition is made to the start-up mode after the predetermined halt time has elapsed after transiting to the halt mode, and wherein the switching controller further comprises: a pulse width modulator which generates a pulse signal, duty ratio of the pulse signal gradually changing in the start-up mode, and being fixed at a predetermined value in the normal mode and the halt mode; a hysteresis comparator which compares the output voltage and a threshold voltage set in a vicinity of a target value of the output voltage and generates an overvoltage detection signal; and a driver circuit which drives the switching transistor based on the pulse signal while the overvoltage detection signal indicates a first level and stops the switching operation of the switching transistor while the overvoltage detection signal indicates a second level in the normal mode, and halts the switching operation of the switching transistor.