1. Field of the Invention
The present invention relates to a step-up apparatus or a DC-DC converter.
2. Description of the Related Art
Generally, a step-up apparatus is constructed by a charge pump circuit. On the other hand, in a liquid crystal display (LCD) apparatus, a positive voltage and a negative voltage are required to maintain the quality of the liquid crystal.
A first prior art step-up apparatus for generating a positive voltage and a negative voltage is constructed by a first level shift circuit for receiving a first clock signal to generate two second clock signals opposite in phase with each other, a K (K=2, 3, . . . )-multiple charge pump circuit for generating the positive voltage of K·VDD using the second clock signals where VDD is a power supply voltage, a second level shift circuit for receiving the first clock signal to generate two third clock signals opposite in phase with each other, and a (−K)-multiple charge pump circuit for generating the negative voltage of −K·VDD using the third clock signals. This will be explained later in detail.
In the above-described first prior art step-up apparatus, however, since the (−K)-multiple charge pump circuit is complex, the step-up apparatus is high in cost.
A second prior art step-up apparatus for generating a positive voltage and a negative voltage is constructed by a level shift circuit for receiving a clock signal to generate two phase-opposite clock signals, a K(K=2, 3, . . . )-multiple charge circuit for generating the positive voltage of K·VDD using the two phase-opposite clock signals, and a (−1)-multiple charge pump circuit for generating the negative voltage of −K·VDD using the positive voltage and the two phase-opposite clock signals. This also will be explained later in detail.
In the above-described second prior art step-up apparatus, the number of circuit elements is decreased to simplify the apparatus. However, since the transistors within the level shift circuit need to have a much higher breakdown voltage than that of the level shift circuits of the above-described first prior art step-up apparatus, the thickness of gate insulating layers of the transistors, the gate length and/or gate width of the transistors need to be large, which would degrade the integration.