To power the data terminal equipment (DTE) side of the isolation boundary and the RS-232 charge pump, there are two choices: the interface can either implement an isolated power supply or harvest power from the RS-232 line. Integrating a power supply adds cost and complexity to the system, which is difficult to justify in low-cost sensing applications.

To implement the second option of harvesting power from the RS-232 port, this reference design uses the flow control lines that are ignored in most embedded applications. The RS-232 specification (when properly implemented on a host computer or adapter cable), keeps the request to send (RTS) and data terminal ready (DTR) lines high when the port is active. As long as the host has the COM port open, these two lines retain voltage. This voltage can vary from 5V to 12V, depending on the driver implementation. The 5V to 12V is sufficient for the use requirements in this design.

The voltage is put through a diode arrangement to block signals from entering back into the pins. The voltage charges a capacitor to store energy. The capacitor releases this energy when the barrier and charge pump pull more current than what is instantaneously allowed. The TPS70933 device is used to bring the line voltage down to a working voltage for the charge pump and isolation device.

The TPS70933 linear regulator is an ultra-low quiescent current device designed for power-sensitive applications. A precision band-gap and error amplifier provides 2% accuracy overtemperature. A quiescent current of only 1µA makes these devices an excellent design for battery-powered, always-on systems that require very little idle-state power dissipation. These devices have thermal-shutdown, current-limit, and reverse current protections for added safety. These regulators can be put into shutdown mode by pulling the EN pin low. The shutdown current in this mode goes down to 150nA (typical).