Design Ideas: October 26,, 1995
The circuit in Fig 1 produces a semiregulated 5V output from an RS-232C port. Unlike a PC-mouse supply or comparable circuits that rely on the DTR and RTS modem-control signals, this circuit operates with a three-wire port, GND, RX, and TX, and obtains power only from the TX line. Except at high duty cycles, the TX line (RCV-232 on the circuit side) remains operational while supplying power. The output current, approximately 8 mA, is sufficient for CMOS µCs and other low-power circuits.
IC1 is a switched-capacitor, charge-pump-voltage converter that can either invert or double an input voltage. The connections in Fig 1 provide a doubler configuration in which the normal input-voltage polarity is reversed. A positive input voltage normally connects between the GND and OUT pins. This circuit connects a negative input between OUT and GND, however. The IC then doubles the negative input voltage in the positive direction, producing a positive output at VDD equal to |VIN|.
Zener diode D1 acts as a shunt regulator that semiregulates VIN to -4.7V. The large values of C1, C2, and C3 support the output voltage during worst-case (all-zero) patterns of transmission. At 9600 baud, for example, an all-zeros transmission causes an output droop of about 0.2V. For lower baud rates, you should substitute a proportionally higher value for C1. (DI #1781)