AN860| Application Note
INTERFACE CIRCUITS
Jul 09, 1998
APPLICATION NOTE 860
Serial-Data Interface Chip Supplies Bipolar Voltages
Some of the interface ICs currently available for serial-data transmission not only operate from low VCC levels (5V or 3.3V); they also generate bipolar dc voltages ( 6.5V to 10V) to support the minimum driver-output levels as specified by EIa/TIa-232. With care, you can steal useful amounts of power from these voltage rails without interfering with the IC's operation. In Figure 1, the IC's switch-mode controller operates with an external inductor, two diodes, and two capacitors to produce 6.5V. FETs Q1 and Q2 ensure start-up for the circuit by disconnecting the load until these switch-mode supply voltages are present. Note that Q1 must be a logic-level device.
Figure 1. For data rates and driver-output loads less than the maximum allowed, the V+ and V- outputs of this serial
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