Conventionally, as a display device, a so-called driving circuit-integrated liquid crystal display device is known in which, for example, a driving circuit for driving pixels of a pixel unit having pixels including liquid crystal cells arrayed in a matrix is integrally formed on a single board where the pixel unit is formed (liquid crystal display panel). In this driving circuit-integrated liquid crystal display device, a level converter circuit is provided which performs level conversion of various timing pulses with a low voltage amplitude of, for example, TTL level, inputted into the panel from a control IC provided outside the panel, to timing pulses with a high voltage amplitude necessary for driving the liquid crystal, and supplying the level-converted timing pulses to the driving circuit.
The driving circuit for driving the pixel unit uses various timing pulses are used such as a vertical start pulse VST and a vertical clock pulse VCK for vertical scanning used when selectively driving the pixels of the pixel unit by row, and a write enable pulse WE for allowing writing of a signal to the pixels of the row selected through the vertical scanning. Since these timing pulses have different timing, the timing pulses are level-shifted by different level converter circuits.
Meanwhile, a liquid crystal display device is adapted for displaying an image by changing the molecular arrangement of liquid crystal on the basis of the presence/absence of an electric field and thus controlling transmission/interception of light. Theoretically, it is a display device of low power consumption type that does not need much electric power for driving. Therefore, it is broadly used as an output display unit for a portable terminal device such as a portable telephone unit or a PDA (personal digital assistant) using a battery as its main power. With respect to the liquid crystal display device for this type of application, reduction in power consumption based on lowering of the driving voltage and lowering of the driving frequency has been under way in order to enable use of the battery for a longer time period after charging it once.
In the above-described driving circuit-integrated liquid crystal display device, since separate level converter circuits for level-shifting various timing pulses are used constantly in the power-on state, an unnecessary DC current is consumed, obstructing reduction in power consumption of the whole driving circuit. Therefore, particularly in consideration of application to a portable terminal device such as a portable telephone unit or a PDA, reduction in power consumption of the liquid crystal display device itself is an important task to achieve in order to further reduce the power consumption of the portable terminal device.
The driving system of the display device, for example, a liquid crystal display device, includes a simple matrix system and an active matrix system. However, the active matrix, which has excellent response characteristics and visibility, has recently been used more often. In the liquid crystal display device of the active matrix system, when driving the liquid crystal display panel, a scanning line of a row (line) to which a signal is to be written is selected and then the signal is supplied from, for example, a driver IC provided outside the panel, thereby writing the signal to pixels decided as driving targets in the matrix.
If the signal line of the liquid crystal panel and the output of the driver IC provided outside the panel for driving the signal line are set in the relation of one-to-one correspondence, an IC driver having outputs of the number corresponding to the number of the signal lines must be prepared and wirings of the corresponding number are necessary for connecting the driver IC and the liquid crystal display panel. In view of this, a selector driving system is recently employed which allocates plural signal lines of the signal lines of the liquid crystal panel as a unit (set) to one output of the driver IC, time-divisionally selects the plural signal lines, and time-divisionally allocates and supplies the output signal of the driver IC to the selected signal lines.
Specifically, in this selector driving system, the output of the driver IC and the signal line of the liquid crystal display panel are set in the relation of one-to-N correspondence (N is an integer equal to or more than 2), and N signal lines allocated to one output signal of the driver IC selected and driven by N time division. By employing this selector driving system, it is possible to reduce the number of outputs of the driver IC and the number of wirings between the driver IC and the liquid crystal display panel to 1/N of the number of signal lines.
If the above-described selector driving system is employed in the so-called driving circuit-integrated liquid crystal display device in which the driving circuit for the pixel unit is integrally formed on a single board where the pixel unit is formed (liquid crystal display panel), a selector circuit for time-divisionally allocating one output signal of the driver IC to N signal lines is provided on the liquid crystal display panel. This selector circuit is switch-controlled (or selection-controlled) by a selector pulse supplied from outside.
Moreover, a level converter circuit for converting a signal with a low voltage amplitude of, for example, TTL level, supplied into the panel from an external IC to a signal with a high voltage amplitude necessary for driving the liquid crystal is provided on the liquid crystal display panel. The selector pulse is with a low voltage amplitude (for example, 0 to 3 V) of TTL level is inputted to the level converter circuit, where the selector pulse is level-shifted to a high voltage amplitude (for example, 0 to 7 V) necessary for driving the liquid crystal and supplied to the selector circuit for time-division control.
In the liquid crystal display device of the above-described selector driving system, conventionally, since the level converter circuit for performing level conversion (level shift) of the selector pulse with an external circuit power-supply voltage to an internal circuit power-supply voltage is used constantly in the power-on state, an unnecessary DC current is consumed, obstructing reduction in power consumption of the whole driving circuit. Therefore, particularly in consideration of application to a portable terminal device such as a portable telephone unit or a PDA, reduction in power consumption of the liquid crystal display device itself is an important task to achieve in order to further reduce the power consumption of the portable terminal device.