Browse Prior Art Database

IMPROVED VERTICAL DRIVER CIRCUIT FOR CCD IMAGE SENSOR

IP.com Disclosure Number: IPCOM000008716D
Original Publication Date: 1998-Jun-01
Included in the Prior Art Database: 2002-Jul-05
Document File: 3 page(s) / 146K

Publishing Venue

Motorola

Related People

Harry David Bush: AUTHOR

Abstract

The vertical drive of the CCD sensor is used to shift the array pixels into a shift register where the horizontal drive circuit shifts them to the video output amplifier. While only one row of pixels is trans- ferred from the array to the shift register, every line in the array must be moved down by the vertical driver. The array requires two complementary drive pulses every line, one starting from a -8.5 volts and shifting to -1.0 volts, while the other starts at -1.0 volts and shifts to -8.5 volts. In addition, these inputs are driven to +8.5 volts to select a field, two fields composing a frame, alternately. All pulses for both horizontal and vertical drivers are generated in a logic array at the appropriate time.

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MO-LA Technical Developments

IMPROVED VERTICAL DRIVER CIRCUIT FOR CCD IMAGE SENSOR

by Harry David Bush

BACKGROUND

  The vertical drive of the CCD sensor is used to shift the array pixels into a shift register where the horizontal drive circuit shifts them to the video output amplifier. While only one row of pixels is trans- ferred from the array to the shift register, every line in the array must be moved down by the vertical driver. The array requires two complementary drive pulses every line, one starting from a -8.5 volts and shifting to -1.0 volts, while the other starts at -1.0 volts and shifts to -8.5 volts. In addition, these inputs are driven to +8.5 volts to select a field, two fields composing a frame, alternately. All pulses for both horizontal and vertical drivers are generated in a logic array at the appropriate time.

  The previous design generated the required voltages from resistor networks, op amps, and power drivers. These required considerable quies- cent current, to meet the pulse rise time require- ments, even when not changing voltage levels.

  Note: The terms "horizontal" and "vertical" are not quite the same as used in television. The Horizontal driver shifts the video out of the shift register while the Vertical driver selects both the next line (the TV horizontal function) and the next field (the TV vertical function) by the voltage

excursion.

THE SOLUTION

  It was noted that the CCD array was almost entirely capacitive and about 0.068 micro farads. Thus, it could act as its own hold capacitor. Instead of driving the array with op amps, the invention uses capacitive dividers and saturating switches to realize the required voltages. Power is dissipated only during transitions; no op amp and power driver quiescent current.

  From the voltage requirements of the array, a pulse of 7.5 volts is required for each new line shifted into the horizontal shift register, and a pulse of 9.5 volts for each new field. The available fixed voltages in the system are +S volts and +I5 volts, for other electronic components. It is obvious that the +5 volt supply is too small to be of use, so the 15 volt supply is used. Transistor switches are driven by the logic array to connect a node to either ground or +15 volts. A capacitor is connected from this node to the vertical input of the CCD array, forming a capacitor voltage divider. The size of this capacitor is calculated to produce the required voltage change on the array.

  A second set of transistor switches is used to generate the 9.5 volt pulse for field selection in a similar manner. When this is done, it was noted that one of the transistor collectors would be reversed biased. To prevent this, a diode was added between the collectors of the switches. Each set of switches required this modification. The capacitor selected for the 9.5 volt pulse is somewhat larger than for the
7.5 volt pulse, as would-be expected.

  The two vertical inputs for the CCD array are DC biased to about -1 ....