Laptop computers often use large-screen LCDs, which require a variable and a negative supply to ensure maximum 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 microproces-sor 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 possble voltages between these limits.the DAC by using the rail-to-rerail output-drive capability of a 74 HC-series CM0S gate. A resistor divider network formed by the 240-kΩ resistor, connected to the -V filter capadtor and the resistors, is refer-enced 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 uA. 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 dis-ables the gate drive to the FET and turns off the internal oscillator.
The MAX639/MAX640/MAX653 step-down switching regulators provide high efficiency over a wide range of load currents, delivering up to 225mA. A current-limiting pulse-frequency-modulated (PFM) control scheme gives the devices the benefits of pulse-width-modulated (PWM) converters (high efficiency at heavy loads), while using only 10µA of supply current (vs. 2mA to 10mA for PWM converters). The result is high efficiency over a wide range of loads.
The MAX639/MAX640/MAX653 input range is 4V to 11.5V, and the devices provide lower preset output voltages of 5V, 3.3V, and 3V, respectively. Or, the output can be user-adjusted to any voltage from 1.3V to the input voltage. The MAX639/MAX640/MAX653 have an internal 1A power MOSFET switch, making them ideal for minimum-component, low- and medium-power applications. For increased output drive capability, use the MAX649/MAX651/MAX652 step-down controllers, which drive an external P-channel FET to deliver up to 5W.
V+...........................................................................................12V
LX .........................................................(V+ - 12V) to (V+ + 0.3V)
LBI, LBO, VFB, SHDN, VOUT........................-0.3V to (V+ + 0.3V)
LX Output Current (Note 1) ......................................................1A
LBO Output Current............................................................10mA
Continuous Power Dissipation (TA = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C) .............727mW
SO (derate 5.88mW/°C above +70°C)..........................471mW
CERDIP (derate 8.00mW/°C above +70°C)..................640mW
Operating Temperature Ranges:
MAX639C_ _ .......................................................0°C to +70°C
MAX639E_ _ ....................................................-40°C to +85°C
MAX639MJA ..................................................-55°C to +125°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
Note 1: Peak inductor current must be limited to 600mA by using an inductor of 100µH or greater.
Required Field 
