The LT1684 is a telecommunication ring tone generator. The IC takes a user-generated pulse width modulated (PWM) input and converts it to a high voltage sine wave suitable for telephone ringing applications.The LT1684 receives capacitor-isolated differential PWM input signals encoded with desired ring output cadence, frequency, and amplitude information. The LT1684 normalizes the pulse amplitude to +/-1.25V for an accurate signal voltage reference. The cadence, frequency and amplitude information is extracted using a multiple-pole active filter/amplifier, producing the output ring tone signal.The LT1684 uses its own ring tone output as a reference for generating local supply rails using complementary high voltage external MOSFETs as dynamic level-shifting devices. This 'active tracking' supply mode of operation allows linear generation of the high voltage ring tone signal, reducing the need for large high voltage filtering elements.
