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LT1684 Datasheet, PDF (10/16 Pages) Linear Technology – Micropower Ring Tone Generator
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LT1684
APPLICATIO S I FOR ATIO
The component selections for the active tracking supply
lowpass MFB filter configuration follow the relations:
C2 = mC1
m ≤ 1 / [4Q2(1+|HO|)]
R2 = 1± [1–4mQ2(1+|HO|)]1/2
2ωnC1mQ
R1 = R2 / |HO|
R3 =
1
ωn2C12R2m
Example:
Conditions: Output ring tone peak voltage = 100V
Ring frequency = 20Hz
Input duty cycle range = 10% to 90%
Filter Q = 0.707
Set:
Choose:
Then:
fn = ωn / 2π = 100Hz
C1 = 1.0µF (a convenient value)
m ≤ [4(0.7)2(1+100)]–1 ≈ .005
C2 = mC1
C2 = 4700pF
(sets m = 0.0047)
R2 = 1± [1– 4(0.0047)(0.707)2(101)]1/2
(4π100)(1e–6)(.0047)(0.707)
R2 = 300k
R1 = 300k/100
R1 = 3.0k
R3 = [(2π100)2(1e–6)2(300k)(0.0047)]–1
R3 = 2k
This filter configuration yields a DC Gain of 100, a corner
frequency of just under 100Hz with gain reduction of only
0.1% at 20Hz, and a 10kHz carrier rejection of greater than
40dB at the output.
Active Tracking Supply Components
Given the previous discussion, implementation of an
active tracking supply system may seem almost trivial.
Active Tracking Supply Lowpass Multiple
Feedback Filter Transfer Characteristic (AV vs fn)
50
0
–50
1
10 100 1K 10K 100K
HERTZ (Hz)
1684 F02
However, bootstrapping an amplifier system about its
own output creates a complex myriad of inherent stability
and response issues. Attempting such a configuration
with generic “jelly-bean” components is not recommended
for the faint of heart or type-A personalities. The LT1684,
however, makes for a simplistic approach to active track-
ing component selection.
The high voltage MOSFET transistors used in the circuit
must have an operating VDS specified at greater than the
corresponding high voltage supply rail plus the opposite
maximum excursion of the output signal. For example, if
a system is designed with a 240V supply (+ 120V,
–120V) and outputs a ring signal that has a 100V peak
amplitude, the MOSFET VDS ratings must be greater than
240/2 + 100 = 220V.
Active Filter Tuned Oscillator—
No PWM Input Required
A simple yet effective method of producing a high quality
sine wave is to place a high-Q bandpass filter and a hard
limited gain element in a positive feedback loop. This
circuit will oscillate at the bandpass frequency, producing
a sine wave at the filter output. The product of the funda-
mental component of the limiter and the filter gain at the
bandpass frequency determines the output amplitude.
This type of circuit is commonly referred to as an active
filter tuned oscillator.
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