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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

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

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

the IN A input is driven higher than IN B (by the required

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

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