Apparatus for driving electric load

#1. Field of the Invention

#This invention generally relates to an apparatus for driving an electric load or a current consumer. This invention particularly relates to an apparatus for driving an electric load such as a coil, an inductor, or a lamp whose internal resistance varies as it is energized.

#2. Description of the Related Art

#FIG. 1 is a diagram of an apparatus for driving an electric load according to a first embodiment of this invention.

#FIG. 2 is a time-domain diagram of the state of a switch, and conditions of voltages and a load current in the apparatus of FIG. 1.

#FIG. 3 is a time-domain diagram of the state of a switch, and conditions of a voltage and a load current which occur in an assumed case.

#With reference to FIG. 1, a driving apparatus according to a first embodiment of this invention turns on and off an electric load, for example, a lamp 2 in response to a command signal fed from an external. The lamp 2 is provided in, for example, a headlight of an automotive vehicle.

#The driving apparatus of FIG. 1 includes an n-channel power MOSFET 6 which acts as a high side switch. The FET 6 is interposed in a power feed line extending from the positive terminal of a battery 4 to one end of the lamp 2. The negative terminal of the battery 4 and the other end of the lamp 2 are grounded. Specifically, the drain of the FET 6 is connected to one end of a current sensing resistor 8. The other end of the current sensing resistor 8 is connected to the positive terminal of the battery 4. The source of the FET 6 is connected to one end of the lamp 2. The other end of the lamp 2 is grounded. A load current generated by the battery 4 flows through the current sensing resistor 8, the drain-source path of the FET 6, and the lamp 2. The resistor 8 senses the load current. Specifically, the resistor 8 causes a signal voltage representing the load current. The load current is controlled in response to a voltage at the gate of the FET 6.

#The driving apparatus of FIG. 1 further includes a trapezoidal wave generation circuit 10 and a load current control circuit 20. The trapezoidal wave generation circuit 10 produces a signal of a trapezoidal waveform in response to a command signal fed from an external. The trapezoidal wave generation circuit 10 outputs the trapezoidal wave signal to the load current control circuit 20. The load current control circuit 20 is connected to the current sensing resistor 8. The load current control circuit 20 receives the signal voltage representative of the load current from the current sensing resistor 8. The load current control circuit 20 is connected to the gate of the FET 6. The load current control circuit 20 compares the voltage of the trapezoidal wave signal and the signal voltage representative of the load current. The load current control circuit 20 adjusts a voltage at the gate of the FET 6 in response to the result of the voltage comparison so that the load current will vary as a trapezoidal waveform corresponding to the trapezoidal wave signal. In other words, the load current control circuit 20 adjusts the load current into waveform agreement with the trapezoidal wave signal on a feedback control basis.