UCC3751

The UCC3751 controller is designed for driving a power stage that generates low frequency, high voltage sinusoidal signals for telephone ringing applications. The controller and the power stage are most suitable for single line applications where low cost, high efficiency and minimum parts count are critical. In addition to providing the sinusoidal ringing signal, the controller and the power stage are designed to provide the required DC voltage across the output when the phone goes off-hook. The DC voltage is also added as the offset to the ringing signal. This feature eliminates the need to have a separate talk battery voltage power supply as well as relays and drivers to switch between the ringing voltage and the talk battery.

The UCC3751 directly drives primary side switches used to implement a push-pull resonant converter topology and transformer coupled sampling switches located on the secondary of the converter. For normal ring signal generation, the primary switching frequency and secondary sampling frequency are precisely offset from each other by the ringing frequency to produce a high voltage low frequency alias signal at the output. The off-hook condition is detected by sensing the AC current and when AC limit is exceeded, the sampling frequency is set to be equal to the primary switching frequency to produce a DC output.

The drive signal frequencies are derived from a high frequency (3579545 Hz) crystal. The primary switching frequency is 89.488 kHz and the sampling frequency is 20, 25 or 50 Hz less depending on the status of frequency select pins FS0 and FS1.