There are some circuits that require a voltage higher than the battery source. For this reason, a voltage doublin g charge pump circuit is provided. U1501 takes the 2.75V

source and oscillates it w ith a high pe ak to peak signal. The positive region of the signal is then sent out to produced the VDOUB supply.

BATT_SAV is used to cut off the VDOUB supply to conserve battery power. When the BATT_SAV line is on (high) , U1501 will b e disabled.

U903 is the ne gative supply generator. It is used to supply -5V to the LCD displ ay contrast circuitry.

U903 generates a negative voltage by generating a 3 Vpp square wave from the 2.75V source. The signal is then rectified to generate a -3V supply. This supply is not at a

suffici ent level for proper operation of the LCD display. In order to increase the negative voltage a sample of a square wave that U903 produces is combined with the -3V

output of U903. This process will change the voltage reference of the square wave to -3V instead of 0V. The square wave is now reaching a negative peak value of -5.4V.

The square wave is then rectified by CR1590, allowing a -5V volt supply to be generated.