1. Field of the Invention
The present invention relates to a semiconductor device for power supply, and, in particular, to a DC-DC converter which is a kind of a switching-type constant-voltage power supply in which, when an externally supplied voltage given by an external supply power source such as a battery or the like is converted into a predetermined constant voltage, and, then, is outputted, determination is made as to whether a condition of a load to which the constant voltage is provided is a heavy load condition or a light load condition, and switching of a switching transistor is controlled in accordance with the load condition so that an improvement in power-supply efficiency is achieved at the time of a light load.
2. Description of the Prior Art
As such a kind of DC-DC converter in the prior art, there is one which is disclosed in Japanese Laid-Open Patent Application No. 7-322608 (see FIG. 1), for example.
Specifically, a DC-DC converter 2A in the prior art, shown in FIG. 1, supplies power, via a common power-supply line 18A, to a heavy-load circuit 12A such as a DC-motor driving circuit or the like and a light-load circuit 15A such as an IC circuit generating a control signal or the like. The DC-DC converter 2A has a switching circuit including a transistor 5A which is inserted in series or in parallel to the heavy-load circuit and light-load circuit and performing switching control of the transistor 5A for the power-supply line 18A, a control circuit 6A which controls periods of switching of the transistor 5A through pulse-width modulation (PWM) control or pulse-frequency modulation (PFM) control according to a voltage V001 at a load side so that this voltage V001 is fixed, and an oscillation circuit 7A which turns on and turns off the transistor 5A in a predetermined period.
In the DC-DC converter having the above-described circuit arrangement, a control unit 20A (MCU) causes a battery voltage Vcc of a battery 16A to be provided to the control circuit 6A via a switch 8A, causing the control circuit 6A to operate so that the control circuit 6A performs the pulse-width modulation control, when power is supplied to the heavy-load circuit 12A. The control unit 20A causes the battery voltage Vcc of the battery 16A not to be provided to the control circuit 6A, causing the control circuit 6A to stop operating, and, simultaneously, causing the battery voltage Vcc of the battery 16A to be provided to the oscillation circuit 7A via the switch 8A, causing the oscillation circuit 7A to operate so that the oscillation circuit 7A performs the pulse-frequency modulation control, when power is supplied only to the light load circuit 15A.
However, in this DC-DC converter 2A in the prior art, because switching is performed such that the pulse-width modulation control or the pulse-frequency modulation control is used according to the load condition, although the efficiency is improved when the switching is performed such that the pulse-frequency modulation control is used at the time of light load, the load-side output direct-current voltage problematically includes a noise having a random frequency.