1. Field of the Invention
The present invention relates to a power supply for positive and negative output voltages, which is enabled to generate a predetermined negative output voltage together with a positive output voltage converted from a power supply voltage by using a switching power circuit that employs a coil.
2. Description of the Related Art
In a related-art, a switching power supply employing a coil as a DC-DC converter for generating a voltage differing from a power supply voltage has been used. In a case where the switching power supply is of, for example, the step-up type, a high voltage is obtained from a DC input voltage by turning on and off electric current, which is fed to a coil, through the use of a switch. Then, the high voltage is rectified and smoothed. Thus, a raised output voltage is obtained (see M. Suzuki, “Teihon Zoku Toransistor Kairo No Sekkei”, CQ Publishing Co., Ltd., Jul. 1, 1997, p. 250–p. 253).
To generate a voltage (a negative output voltage) of a polarity differing from a power supply voltage, a negative output voltage is generated from a power supply voltage by using a negative output voltage charge pump circuit. In a case where the voltage is changed to a constant voltage, it is necessary to obtain a predetermined negative output voltage from the generated negative output voltage by using a series regulator or the like (see JP-A-2000-91503).
In recent years, the number of electronic devices (for instance, CCD cameras), which individually require a predetermined positive output voltage power supply and another predetermined negative output voltage power supply, has been increased. However, a related switching power supply circuit, which employs a coil and is generally used as a DC-DC converter, can get only a positive output voltage. When not only a positive output voltage but a predetermined negative output voltage is needed, a negative output voltage obtained by the combination of the related negative output voltage charge pump circuit and the series regulator has been additionally provided.
In the case that a positive output voltage power supply circuit and a negative output voltage power supply circuit are separately provided, the number of components increases, and a required space increases. Also, the related art has a problem that the cost increases.