Simple active-matrix-LCD bias supply operates from battery input

Michael Shrivathson, National Semiconductor Corp, Santa Clara, CA -- 2/3/2000

Many active-matrix-LCD applications need multiple voltages for thin-film-transistor (TFT) bias. Typically, three voltages are necessary: 5V for the column driver; a positive voltage, such as 10V; and a negative voltage, such -5V, for the TFT gate drive, or row driver. For handheld electronic devices, a battery must produce these voltages. The most popular batteries in these devices are two-cell NiCd alkaline or one-cell lithium-ion batteries.

Figure 1 shows a simple, cost-effective way of providing these bias voltages. A step-up regulator, IC₁, forms the heart of the circuit. This regulator switches at a constant frequency of 1 MHz and a fixed duty cycle of 70%. IC₁ steps up the input voltage to 5V by storing the energy in the inductor when the internal MOSFET, M₁, is on and transferring this energy to C₁ when M₁ is off. IC₁'s hysteretic gated-oscillator control scheme achieves the regulation.

C₂, C₃, D₂, and D₃ form a charge-pump inverter to provide an output of approximately -5V. When M₁ is off, C₂ connects in parallel with C₁ through D₁ and D₂. Thus, C₂ charges to V_COL, or 5V. When M₁ turns on, C₂ connects in parallel with C₃ through M₁ and D₃. Because of the polarity of this connection, C₃ charges to approximately -V_COL, or -5V.

C₄, C₅, D₄, and D₅ form a charge-pump doubler that provides an output of 10V. When M₁ is on, C₄ connects in parallel with C₁ through D₄ and M₁. Thus, C₄ charges to V_COL (5V). When M₁ turns off, C₁ and C₄ connect in series through D₁ and D₅, and this series pair connects in parallel with C₅. Thus, C₅ charges to approximately two times V_COL, or 10V.

This circuit provides 250 mA at the 5V output, V_COL, with 3% accuracy. The ac ripple is less than 100 mV. The circuit regulates the 10V output, V_GATE(+), with 5% accuracy, and this output can provide 10 mA. The ac ripple at the 10V output is approximately 30 mV. The circuit regulates the -5V output, V_GATE(-), with 6% accuracy and provides as much as 10 mA of output current. The ac ripple voltage at this output is 40 mV. A minimum load of 25 mA at the V_COL output ensures sufficient charge-pump action and thus maintains V_GATE(+) and V_GATE(-) at their nominal values. The efficiency of this circuit varies from 75 to 82% when operating from a one-cell lithium-ion battery. (DI #2477)

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