5_V_SUPPLY_FROM_THREE_WIRE_RS232_PORT

Published:2009/7/12 22:14:00 Author:May | From:SeekIC

This circuit produces a semiregulated 5-V output from an RS232 port. The output current-about 8 mA-is sufficient for CMOS microcontrollers and other low-power circuits. IC1 is a switched-capacitor, charge-pump voltage converter that can either invert an input voltage or double it. The connections shown provide a doubler configuration in which the normal input voltage is reversed: A positive input voltage normally connects between GND and OUT, but this circuit connects a negative input between OUT and GND. The IC then doubles the negative Vin the positive direction, producing a positive output (at VDD) equal to [Vin]. The zener diode D1 acts as a shunt regulator that semiregulates Vin to -5 V (actually to -4.7 V). The 33-μF capacitor values shown are larger than normal to support the oqtput voltage during worst-case (all-zero) patterns of transmission. At 9600 baud, for example, an all-zeros character causes an output droop of about 0.2 V. For lower baud rates, substitute a proportionally higher value for C1.



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