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 BG
OUT
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 AT
REF
. 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 BG
OUT
. When
the IN A input is driven higher than IN B (by the required
1.6V), the reference drives BG
OUT
to +1.25V above OUT.
When IN B input is driven higher than IN A, BG
OUT
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 BG
OUT
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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