LCD_AUXILIARY_BIAS_CIRCUIT

Published:2009/7/16 23:11:00 Author:Jessie | From:SeekIC

This single-cell boost converter can generate the low supply voltages commonly needed in pagers and other portable instruments with small, graphic LCDs. The first is a regulated 3.3 V at 100 mA, and the other is a regulated negative output, suitable for use as an LCD bias voltage. The overall efficiency is about 80 percent. The main 3.3-V supply is provided by a boost converter (IC1). The auxiliary bias voltage is provided by an extra flyback winding (the T1 secondary), and is regulated via Q1 and the low-battery detector internal to IC1. As the battery discharges, its declining terminal voltage causes a decline in the voltage in the flyback winding. At minimum battery voltage (0.8 V), the T1 primary sees 3.3 V-0.8 V2.5 V, so the 6:1 turns ratio produces 6(2.5)=15 V in the secondary. At maximum battery voltage (1.65 V), the primary sees only 1.5 V, producing 9.9 V in the secondary. MOSFET Q1 stabilizes this output by interrupting the secondary current, introducing the regulation necessary to generate a constant negative output. The regulator uses IC1's low-battery detector (a comparator/reference combination) as an on/off controller for Q1. In this circuit, the R1/R2 divider holds LB1 between Vctrl (normally 3.3 V) and the LCD bias output (normally -8 V). R1 and R2 are chosen so that LBO turns off when the LCD bias becomes too negative (and pulls the LB1 voltage below 1.25 V). Load current then causes the LCD bias to drift upward (toward 0 V) until LB1 exceeds 1.25 V, which causes Q1 to turn on again. A logic signal at the LCD ON terminal provides a means to enable and disable the negative output.



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