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LT1684IS Datasheet(PDF) 7 Page - Linear Technology |
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LT1684IS Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LT1684 BASIC THEORY OF OPERATION The LT1684 operates using a user-provided pulse-width- modulated (PWM) digital signal as input*. The low fre- quency modulation component of this signal represents the desired output waveform. Changing the PWM input can thus dynamically control the frequency, cadence, amplitude and DC offset of the desired output. This method of sine wave generation can accomodate all popular ring tone frequencies including 17Hz, 20Hz, 25Hz and 50Hz. The LT1684 receives the PWM input by a capacitor- isolated differential input at pins IN A and IN B. This signal is amplitude normalized by a bandgap reference and output single-ended on the BGOUT pin such that the PWM carrier is ±1.25V about the voltage on the OUT pin. The low frequency component of the normalized PWM signal is recovered using an active filter circuit con- structed using an onboard driver amplifier. This amplifier also provides current drive for the final ring tone output. The ring tone output is used as the reference for a floating active biasing scheme by pin ATREF. As the ring tone output rises and falls through its typical range of hundreds of volts, the LT1684 “tracks” the output signal, maintain- ing local supply voltages across the IC of approximately ±10V. Input Receiver/Reference Buffer The differential receiver for the PWM input signal requires minimum differential input levels of 1.6V to assure valid change-of-state. The receiver inputs are capacitor coupled, isolating the LT1684 from the PWM generator. The re- ceiver is leading edge triggered. The input receiver controls a switched-state output that forces an amplitude normalized voltage (referenced to the OUT pin) of ±1.25V that follows the PWM input. This switched voltage is driven off-chip on pin BGOUT. When the IN A input is driven higher than IN B (by the required 1.6V), the reference drives BGOUT to +1.25V above OUT. When IN B input is driven higher than IN A, BGOUT is forced to –1.25V relative to OUT. The amplitude normalized representation of the input PWM signal is used as the input for the active filter element and output driver. Output Amplifier/Driver The normalized PWM signal output on the BGOUT pin is converted to the final ring tone signal by an active filter. This filter consists of an onboard amplifier and a few external components. Although many different types of filters can be constructed, a 2-pole Multiple Feedback (MFB) configuration generally provides adequate perfor- mance and is desirable due to its simplicity and effective- ness. The low frequency component of the ±1.25V PWM signal contains the desired ring tone frequency and cadence information. The MFB active filter strips this information from the PWM signal and amplifies this low frequency component to generate the final desired output. Active Tracking Supplies Implementation of the active tracking supply technique enables linear generation of the ring tone output, and takes advantage of the intrinsic supply noise immunity of a linear amplifier, reducing the need for large high voltage filtering elements. Two external power MOSFETs act as voltage level-shifting devices and generate the power supply voltages for the LT1684. The LT1684 uses its own output as a voltage reference for the FET level shifters, “suspending” itself (by these generated supply voltages) about the signal output. In this manner, the LT1684 can linearly generate a signal hundreds of volts in amplitude at its output, while main- taining ±10V local supply rails across the IC itself. (Refer to Functional Block Diagram) * Contact Linear Technology for code. OPERATIO |
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