STEP_UP_SWITCHINC_IC_INVERTER

Published:2009/7/16 20:56:00 Author:Jessie | From:SeekIC

A popular step-up switching IC can be made to supply a negative voltage. The MAX641BCPA is a boost converter intended for use with 3-V batteries. To get the 641 to function as an inverter, some of its pin functions must be altered. Pin 1 is used as the voltage-feedback input, and pin 2 is used as the overvoltage-detector output. When the voltage on pin 1 falls below + 1.31 V, pin 2 becomes a low-impedance path to ground. This will force Q1 to cut off and allow the voltage on pin 7 to rise. When the voltage on pin 7 exceeds +1.31 V, the internal oscillator shuts off. At this point, the output voltage is equal to or less than the set level. When C1 discharges to a point where the voltage on pin 1 rises above +1.31 V, pin 2 will change to a high impedance, allowing Q1 to saturate and pull pin 7 below +1.31 V. This causes the internal oscillator to turn on. Pin 4, the output of the internal oscillator, will pump C1 negative while the oscillator is running. For the values shown in the figure, the adjustable output voltage range is approximately -4 to -13 V. The magnitude of the drivable load depends on how much ripple can be tolerated. The values shown will easily drive a 50- to-75-mA load and provide good regulation.



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