The following figure’s switching regulator generates a negative voltage by the notebook battery supply. The microprocessor data bus drives a 4-bit DAC( 74HC273 ), which in turn can vary the regulator output between 6.5 to 11.5 V. This arrangement enables a staircase of 16 possible voltages between these limits. The LCD power supply circuit implements the DAC by using the rail-to-rail output-drive capability of a 74 HC-series CMOS gate. A resistor divider network made by the 240-kO resistor, connected to the V filter capacitor as well as the resistors, is referenced to the 5 V supply control (the MAX635 regulator).
