MAXIM MAX232 / MOTOROLA MC145407 Transmitter Voltage Doubler-Inverter Switched-Capacitor Model

MAX232 and MC145407 are single-supply CMOS RS-232 transceivers. MAXIM's MAX232 is a dual driver/receiver with derivative devices having various numbers of drivers and receivers. MOTOROLA's MC145407 is available only as triple driver/receiver. LINEAR TECHNOLOGY's LT1080 is a similar device but uses bipolar transistors. The circuit is a switched-capacitor model of voltage doubler-inverter employed in the transmitter section of these RS-232 transceivers. Since the inverter loads the doubler, the doubler voltage has a sharp droop rate but is replenished quickly at succeeding clock edge. The inverter output voltage, on the other hand, droops slowly. Values of charge pump flying and reservoir capacitors vary depending on output loading. Typically it ranges from 1 to 10 µF. With heavy loading some will use even higher values. Variants of MAX232 and LT1080 that use higher oscillator frequency can use as low as 0.1 µF. Charge Pump Oscillator Frequencies: MAX202E 230 kHz MAX232E 140 kHz MC145407 20 kHz LT1130, LT1180 180 kHz