1. Field of the Invention
The present invention relates to a read controller for an image pick-up device that controls reading of charges accumulated in a plurality of sensors disposed in the image pick-up device through predetermined transfer paths and to an image pick-up apparatus that captures object light by the image pick-up device and creates an image signal.
2. Description of the Related Art
A digital camera as one of image pick-up apparatuses widely employs a charge coupled device (hereinafter, abbreviated as CCD) as an image pick-up device. The CCD is the image pick-up device having photoelectric transducers such as photo diodes and the like as a plurality of sensors disposed on a semiconductor substrate, wherein the photoelectric transducers accumulate charges according to an amount of light, and the accumulated charges are read out through predetermined transfer paths.
A substrate bias voltage is applied to the semiconductor substrate constituting the CCD from a bias controller disposed inside or outside the CCD, and the photoelectric transducers have a potential profile for accumulating charges according to the magnitude of the substrate bias voltage applied thereto. Further, the CCD often includes an over drain structure to which an overflow drain voltage (hereinafter, abbreviated as OFD voltage) is applied to discharge the saturated charges accumulated in the photoelectric transducers to the semiconductor substrate. The CCD arranged as described above is controlled such that excessive charges do not overflow to the transfer paths even if pixels mixedly exist in the CCD when a moving picture is recorded by, for example, decreasing the OFD voltage when a still picture is recorded to thereby accumulate a large amount charges in the photoelectric transducers and by increasing the OFD voltage when the moving picture is recorded to thereby accumulate a small amount of charges therein. Further, there are proposed in the CCD a technique for adjusting the OFD voltage by a load means connected to a control terminal (refer to, for example, Japanese Patent Application Laid-Open No. 9-121310) and a technique for making the amount of saturated charges accumulated in the photoelectric transducers uniform by setting a potential barrier in a high state by applying a predetermined OFD voltage (refer to, for example, Japanese Patent Application Laid-Open No. 11-331708).
There are known digital cameras provided with a mechanical shutter as well as a read controller for an image pick-up device that controls reading of charges from a CCD having the over drain structure. When a still picture is picked up with the digital camera, first, exposure is executed. In the exposure, there are executed processing for displaying an object observing through image on an image monitor in the state that the mechanical shutter is opened, and the like. Next, smear is read out to clear transfer paths by reading out the charges remaining in the transfer paths in the state that no light is incident by closing the mechanical shutter. The smear is read out in response to relatively high speed pulses. In general, when the still picture is picked up, the OFD voltage is set to a relatively low level so that charges are sufficiently accumulated in the photoelectric transducers. Accordingly, spike-like noise occurs in the OFD voltage set to the low level because the OFD voltage is affected by the high speed pulses in the smear read-out process. The noise is mixed with the substrate bias voltage that determines the magnitude of the potential profile of the photoelectric transducers. As a result, a problem arises in that the charges accumulated in the photoelectric transducers are discharged to a semiconductor substrate, and the amount of saturated charges of the photoelectric transducers decrease. In particular, when a power supply having a small current capacity is used, this problem arises outstandingly. Accordingly, it is difficult to obtain an image of excellent quality by the digital camera as an image pick-up apparatus.