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PDF LT1684 Data sheet ( Hoja de datos )
| Número de pieza | LT1684 | |
| Descripción | Micropower Ring Tone Generator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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LT1684
Micropower
Ring Tone Generator
FEATURES
s Allows Dynamic Control of Output Frequency,
Cadence, Amplitude and DC Offset
s Active Tracking Supply Configuration Allows Linear
Generation of Ring Tone Signal
s No High Voltage Post-Filtering Required
s Capacitive Isolation Eliminates Optocouplers
s Low Distortion Output Meets International
PTT Requirements
s Differential Input Signal for Noise Immunity
s User Adjustable Active Output Current Limit
s Powered Directly From High Voltage Ringer
Supply—No Additional Supplies Necessary
s Supply Current: < 1mA
s 2% Signal Amplitude Reference
s Available in 14-Pin SO and DIP Packages
U
APPLICATIO S
s Wireless Local Loop Telephones
s Key System/PBX Equipment
s Fiber to the Curb Telecom Equipment
DESCRIPTIO
The LT®1684 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 nor-
malizes 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
Electrically Isolated Ring Tone Generator
DC
PWM
ISOLATION
CONTROLLER
100pF
P1
10k
µC
P2
100pF
10k
IN A GATE+
IN B V+
LT1684
LIM+
OUT
BGOUT
ATREF
COMP1
FB1
100pF
6.8nF 100k
100Ω
1N4001
3k
2k
300k
1µF
5k
AMPIN
4700pF
COMP2
LIM–
V–
GATE–
20pF
1N5817
100Ω
6.8nF 100k
IRF610
100V
+
RING TONE
OUTPUT
0.1µF
–
( )±100mA
CAPABILITY
IRF9610
–100V
FB1: FERRONICS FMB1601
(716) 388-1020
1684 TA01
1
1 page  
LT1684
PI FU CTIO S
IN B (Pin 1): PWM Negative Input. Input is isolated from
digital source by ~100pF series capacitor. A 10k resistor
can be connected to the IN B pin in series with the isolation
capacitor for transient protection. The PWM receiver imple-
ments a diode forward drop of input hysteresis (relative to
IN A). This hysteresis and internal signal limiting assure
common mode glitch rejection with isolation capacitor
mismatches up to 2:1. For maximum performance, how-
ever, effort should be made to match the two PWM input
isolation capacitors. Pin IN B is differentially clamped to
pin IN A through back-to-back diodes. This results in a
high impedance differential input through ±100mV be-
yond the input thresholds. 5k internal input resistors yield
a 10k (nominal) differential overdrive impedance.
COMP1 (Pin 2): Output Amplifier Primary Compensation.
Connect a 100pF capacitor from pin COMP1 to pin OUT.
COMP2 (Pin 3): Output Amplifier Secondary Compensa-
tion. Connect a 20pF capacitor from pin COMP2 to pin
OUT.
LIM – (Pin 4): Output Amplifier Current Sink Limit. Pin
implements IOUT • R = VBE current clamp. Internal clamp
resistor has a typical value of 3.5Ω. For maximum current
drive capability (190mA typical) short pin to pin V –.
Reduction of current sink capability is achieved by placing
additional resistance from pin LIM – to pin V –. (i.e. An
external 3.5Ω resistance from pin LIM – to pin V – will
reduce the current sinking capability of the output ampli-
fier by approximately 50%.)
V – (Pin 5): Local Negative Supply. Typically connected to
the source of the active tracking supply P-channel MOSFET.
V – rail voltage is GATE – self-bias voltage less the MOSFET
VGS. Typical P-channel MOSFET characteristics provide
ATREF – V – ≈ 10V.
GATE– (Pin 6): Negative Power Supply FET Gate Drive. Pin
sources current from pull-down resistor to bias gate of
active tracking supply P-channel MOSFET. Self-biases to
a typical value of –14V, referenced to pin ATREF. Pull-down
resistor value is determined such that current sourced
from the GATE – pin remains greater than 50µA at mini-
mum output signal voltage and less than 10mA at maxi-
mum output signal voltage.
ATREF (Pin 7): Active Tracking Supply Reference. Typi-
cally connected to pin OUT. Pin bias current is the differ-
ence between the magnitudes of GATE + pin bias and
GATE– pin bias (IATREF = IGATE+ – IGATE–).
OUT (Pin 8): Ring Tone Output Pin. Output of active filter
amplifier/buffer. Used as reference voltage for internal
functions of IC. Usually shorted to pin ATREF to generate
reference for active tracking supply circuitry. Connect a 1A
(1N4001-type) diode between V + and OUT and a
1A Schottky diode from V – to OUT for line transient
protection.
LIM+ (Pin 9): Output Amplifier Current Source Limit. Pin
implements IOUT • R = VBE current clamp. Internal clamp
resistor has a typical value of 3.5Ω. For maximum current
drive capability (190mA typical) short pin LIM + to pin
OUT. Reduction of current source capability is achieved by
placing additional resistance from pin LIM + to pin OUT.
(i.e. An external 3.5Ω resistance from pin LIM + to pin OUT
will reduce the current sourcing capability of the output
amplifier by approximately 50%.)
V + (Pin 10): Local Positive Supply. Typically connected to
the source of the active tracking supply N-channel MOSFET.
This condition should be made using a ferrite bead.
Operating V + rail voltage is GATE + self-bias voltage less
the MOSFET VGS. Typical N-channel MOSFET characteris-
tics provide V + – ATREF ≈ 10V.
GATE + (Pin 11): Positive Power Supply FET Gate Drive.
Pin sinks current from pull-up resistor to bias gate of
active tracking supply N-channel MOSFET. Self-biases to
a typical value of 14V, referenced to pin ATREF. Pull-up
resistor value is determined such that sink current into
GATE + pin remains greater than 50µA at maximum output
signal voltage and less than 10mA at minimum output
signal voltage.
AMPIN (Pin 12): Output Amplifier Input. Connected to
external filter components through series protection re-
sistor (usually 5k). Thevenin DC resistance of external
filter and protection components should be 10k for opti-
mum amplifier offset performance. See Applications In-
formation section.
5
5 Page 
LT1684
APPLICATIO S I FOR ATIO
Active Filter Tuned Oscillator Block Diagram
1684 F03
The LT1684 can be implemented easily into a telephone
ringer circuit based on the active filter tuned oscillator
topology, eliminating the need for a user-supplied PWM
input signal. The LT1684’s active filter amplifier can be
used as a high-Q bandpass filter element by configuring
it as an active tracking supply bandpass. The LT1684’s
controlled output receiver/buffer is also convenient for
use as the hard limiter. Because the LT1684 receiver/
buffer requires a true differential input for proper opera-
tion, a dual comparator IC such as the LT1017 must be
bootstrapped along with the LT1684 to provide differen-
tial control signals. The LT1017 and LT1684 receiver/
buffer combine to create a high gain hard limiter whose
Bandpass MFB Filter
CF2
RF1
+
VIN
–
CF1
RF2
RF3
–
+
VOUT
1684 F4a
Active Tracking Bandpass MFB Filter
RF1
RF2
CF1
– VIN +
–
+
CF2 RF3
VOUT
1684 F5b
output is controlled to ±1.25V. The LT1684 active
bandpass filter is then connected as a positive feedback
element with the limiter component, which completes
the active filter tuned oscillator topology.
The active bandpass filter circuit is easily configured using
a basic MFB bandpass configuration, however, the active
tracking supply technique used by the LT1684 requires
“transformation” of this topology. This “transformation”
swaps the amplifier signal polarity, references all signals
to the output, and references all feedback elements to
ground as described previously in the Filter Design and
Component Selection section.
The design equations for the active tracking bandpass
filter are the same as the pretransformation MFB topology,
such that if CF1 = CF2 = C:
RF1 = Q/(ωO • C •H0)
RF2 = Q/(2Q2 –H0)(ωO • C)
RF3 = 2Q/(ωO • C)
Example:
Conditions: Output peak voltage = 95V
Ring frequency = 20Hz
Bandpass Q = 9.4
A square wave with peak amplitude A has a fundamental
component with amplitude 4A/π, where A = 1.25V. There-
fore, the desired filter’s bandpass gain HO = 95/(4 •
1.25/π) ~ 60. Given capacitor values C = 0.22µF (a conve-
nient value) and desired filter characteristics of: Q = 9.4,
HO = 60, ωO = 2π(20Hz), then: RF1 = 5.6k, RF2 = 2.7k,
RF3 = 680k. The amplitude, frequency and envelope re-
sponse time of the output signal can be adjusted by simply
changing the values of resistors RF1 to RF3 accordingly.
This produces a high voltage, high quality 20Hz sine wave
at the filter output with a peak amplitude of 95V. Differen-
tial amplitude and frequency characteristics are achieved
by simply changing a few resistor values. The output of the
LT1684 is internally current limited to a minimum of
±100mA peak, allowing this ring tone generation circuit to
be used with loads up to 7 REN with no degradation of the
output waveform.
11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet LT1684.PDF ] | |
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