Simple Lcd Display Power Supply

  
Laptop computers often use large-screen LCDs, which require a variable and a negative supply to ensure maxi
Simple Lcd Display Power Supply - schematic

mum contrast. This circuit operates from the system"s positive battery supply and generates a digitally variable negative voltage to drive the display. This figure"s switching regulator creates a negative voltage from the battery supply. The microprocessor data bus drives a 4-bit DAC, which in turn varies the actual regulator output from - 6.5 to -11.5 V. This arrangement allows a staircase of 16 possible voltages between these limits. The circuit implements the DAC by using the rail-to-rail output-drive capability of a 74 HC-series CMOS gate. A resistor divider network formed by the 240-kfi resistor, connected to the -V filter capacitor and the resistors, is referenced to the 5-V supply control (the MAX635 regulator). When the voltage at the VFb pin is greater than ground, the switching regulator turns on. The inductor dumps this energy into the -V filter capacitor. When the voltage at VFb is less than ground, the regulator skips a cycle. The MAX635 regulates the voltage at the junction of the resistor divider to 0 V. Thus, any resistor that the DAC connects to ground (logic 0) will not contribute any current to the ladder. Only the resistors that are at 5 V (logic 1) will be part of the voltage-divider equation. The entire switching-regulator supply draws less than 150 . You can place the circuit in an even lower power mode by interrupting the ground pin. The high-current path is from the battery input through the internal power PMOSFET to the external inductor. Disconnecting the ground connection simply disables the gate drive to the FET and turns off the internal oscillator.




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