Simple active-matrix-LCD bias supply operates from battery input
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 VCOL, 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 -VCOL, 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 VCOL (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 VCOL, or 10V.
This circuit provides 250 mA at the 5V output, VCOL, with 3% accuracy. The ac ripple is less than 100 mV. The circuit regulates the 10V output, VGATE(+), 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, VGATE(-), 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 VCOL output ensures sufficient charge-pump action and thus maintains VGATE(+) and VGATE(-) 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)