Patent ID: 7285939

Claim:
A DC-DC converter for an electric load comprising: a DC power supply; a coil connected to the DC power supply; a switching element that is connected to the coil and controls a coil current supplied to the coil from the DC power supply by performing switching operations to produce a boosted output voltage for the electric load; a switching element driving signal generating circuit that generates a switching element driving signal for switching the switching element based on the coil current and the output voltage; a switching frequency varying circuit that changes a switching frequency of the switching element driving signal with passage of time regardless of the output voltage; a first ON signal generating circuit that determines a first ON time of the switching element based on a result of comparison between the coil current and a specified current threshold value, while generating a first ON signal corresponding to the first ON time; a second ON signal generating circuit that determines a second ON time of the switching element based on a result of comparison between the output voltage and a specified voltage threshold value, while generating a second ON signal corresponding to the second ON time; and an OFF signal generating circuit that generates an OFF signal corresponding to a specified OFF time of the switching element, wherein the switching element driving signal generating circuit generates the switching element driving signal based on the first ON signal, the second ON signal and the OFF signal, the switching frequency varying circuit changes the specified OFF time, the OFF signal generating circuit generates an OFF signal corresponding to the specified OFF time based on a result of comparison between a voltage of a capacitor charged with a specified charging current and a specified OFF time threshold value, and the switching frequency varying circuit changes the specified charging current with the passage of time.