Telephone line circuit DC-to-DC converter

A unique, low-power, DC-to-DC switching converter has been developed to provided DC power for analog telephone lines. The converter uses a flyback configuration with a nonlinear element in one of two separate feedback loops. The feedback provides a dual mode of operation with constant voltage output for low-load currents and limited power output for high-load currents. This allows a simple, versatile circuit configuration that is well suited to telephone-line-feed applications. Its output characteristics cannot be reproduced with conventional linear techniques or switching converter techniques. The authors provide background information on the design of the converter and describe the operation of the converter at the functional block level. They describe the overall circuit features and characteristics of the line-feed application as well as circuit implementation and practical considerations. Finally, laboratory test results are presented