Low Voltage Driving for TFT/LCD

Low Voltage Driving for TFT/LCD

       Disclosed is a circuit which makes it possible to drive a
liquid crystal display addressed by thin film transistors (TFT/LCD)
by low operating voltage.  This circuit comprises a capacitor between
a source electrode and a gate electrode of the TFT, a storage
capacitor with voltage-dependent characteristics and a scheme for
driving gate lines so that an amplitude of gate pulses changes at
each frame time.

      Fig. 1 shows an equivalent circuit model for a pixel of TFT/LCD
array.  A capacitor (Cgs) 8 is a parasitic capacitance between the
source electrode and the gate electrode 1 of TFT 7.  A charge
generated by the steep change of gate voltage is transferred from the
gate electrode to the source electrode through this capacitor.  A
capacitor (Cg) 9 is fabricated additionally so that the optimum
charge can be transferred from the gate electrode to the source
electrode.  A storage capacitor (Cs) 5 is constructed between the
source electrode and a common electrode 3 for all the storage
capacitors.

      Fig. 2 shows the characteristics of voltage dependence of Cs.

      Fig. 3 shows a driving scheme for this TFT/LCD.  The gate lines
are scanned sequentially.  Data voltage is synchronized with gate
voltage and changes at each frame so that AC voltage can be applied
to the liquid crystal cell. When the gate voltage is Vg1, the data
voltage is loaded to the source electrode through the transistor.
When the gate pul...