BELLING BL34018

BL34018
1. GENERAL DESCRIPTION
The BL34018 Speakerphone integrated circuit Incorporates the necessary amplifiers ,
attenuators , and control functions to produce a high quality hands free speakerphone
system.Include are a microphone amplifier,a power audio amplifier for the speaker,transmit and
receive attenuators,a monitoring system for background sound level,and an attenuation control
system which responds to the relative transmit and receive levels as well as the background
level.Also included are all necessary regulated voltages for both internal and external circuitry,
allowing line-powered operation (no additional power supplies required).A chip select pin allows
the chip to be powered down when not in use.A volume control function may be implemented
with an external potentiometer.BL34018 applications include speakerphones for household and
business use,intercom systems,automotive telephones,and others.
2. FEATURE
Voice switched speakerphone circuit.
All necessary level detection and attenuation controls for a hand-free telephone
included.
Background noise level monitoring with long time constant.
Background sound level compensation for transmit and receive levels as well as
the background level.
Wide operating dynamic range through signal compression.
On-chip supply and reference voltage regulation.
Power audio amplifier for typical 100mW output (into 25 ohms) with peak
limiting for speaker to minimize distortion.
Chip Select pin for active/stand by operation.
Linear Volume Control Function.
3. BLOCK DIAGRAM
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4. PIN CONFIGURATIONS
Pin
Name
Description
1
RR
A resistor to ground provides a reference current for the transmit and receive attenuators.
2
RTX
A resistor to ground determines the nominal gain of the transmit attenuator. The transmit
channel gain is inversely proportional to the RTX resistance.
3
TXI
Input to the transmit attenuator. Input resistance is nominally 5.0 k ohms.
4
TXO
Output of the transmit attenuator. The TXO output signal drives the input of the transmit
level detector, as well as the external circuit which drives the telephone line.
5
TLI
Input of the transmit level detector. An external resistor ac coupled to the TLI pin sets the
detection level. Decreasing this resistor increases the sensitivity to transmit channel
signals.
6
TLO
Output of the transmit level detector. The external resistor and capacitor set the time the
comparator will hold the system in the transmit mode after speech ceases.
7
RLI
Input of the receive level detector. An external resistor ac coupled to the RLI pin sets the
detection level. Decreasing this resistor increases the sensitivity to receive channel
signals.
8
RLO
Output of the receive level detector. The external resistor and capacitor set the time the
comparator will hold the system in the receive mode after speech ceases.
9
MCI
Microphone amplifier input. Input impedance is nominally 10 k ohms and the dc bias
voltage is approximately equal to VB.
10
MCO
Microphone amplifier output. The mic amp gain is internally set at 34 dB (50V/V).
11
CP1
A parallel resistor and capacitor connected between this pin and VCC holds a voltage
corresponding to the background noise level. The transmit detector compares the CP1
voltage with the speech signal from CP2.
12
CP2
A capacitor at this pin peak detects the speech signals for comparison with the
background noise level held at CP1.
13
XDI
Input to the transmit detector system. The microphone amplifier output is ac coupled to
the XDI pin through an external resistor.
14
SKG
High current ground pin for the speaker amp output stage. The SKG voltage should be
within 10 mV of the ground voltage at Pin 22.
15
SKO
Speaker amplifier output. The SKO pin will source and sink up to 100 mA when ac
coupled to the speaker. The speaker amp gain is internally set at 34 dB (50V/V).
16
V+
Input dc supply voltage. V+ can be powered from Tip and Ring if an ac decoupling
inductor is used to prevent loading ac line signals. The required V+ voltage is 6.0 to 11 V
(7.5 V nominal) at 7.0 mA.
17
AGC
A capacitor from this pin to VB stabilizes the speaker amp gain control loop and
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additionally controls the attack and decay time of this circuit. The gain control loop limits
the speaker amp input to prevent clipping at SKO. The internal resistance at the AGC pin
is nominally 110 k ohms.
18
CS
Digital chip select input. When at a logic “0” (<0.7 V) the VCC regulator is enabled.
When at a logic “1” (>1.6 V), the chip is in the standby mode drawing 0.5 mA. An open
CS pin is a logic “0”. Input impedance is nominally 140 k ohms. The input voltage
should not exceed 11 V.
19
SKI
Input to the speaker amplifier. Input impedance is normally 20 k ohms.
20
VCC
A 5.4 V regulated output which powers all circuits except the speaker amplifier output
stage. VCC can be used to power external circuitry such as a microprocessor( 3.0 mA
max). A filter capacitor is required. The BL34018 can be powered by a separate regulated
supply by connecting V+ and VCC to a voltage between 4.5 V and 6.5 V while
maintaining CS at a logic “1”.
21
VB
An output voltage equal to approximately VCC/2 which serves as an analog ground for
the speakerphone system. Up to 1.5 mA of external load current may be sourced from VB.
Output impedance is 250 ohms. A filter capacitor is required.
Gnd
Ground pin for the IC (except the speaker amplifier).
23
XDC
Transmit detector output. A resistor and capacitor at this pin hold the system in the
transmit mode during pauses between words and phrases. When the XDC pin voltage
decays to ground the attenuators switch from the transmit mode to the idle mode. The
internal resistor at XDC is nominally 2.6 k ohms.
24
VLC
Volume control input. Connecting this pin to the slider of a variable resistor provides
receive mode volume control. The VLC pin voltage should be less than or equal to VB.
25
ACF
Attenuator control filter. A capacitor connected to this pin reduces noise transients as the
attenuator control switches levels of attenuation.
26
RXO
Output of the receive attenuator. Normally this pin is ac coupled to the input of the
speaker amplifier.
27
RXI
Input of the receive attenuator. Input impedance is nominally 5.0 k ohms.
28
RRX
A resistor to ground determines the nominal gain of the receive attenuator. The receive
channel gain is directly proportional to the RRX resistance.
22
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5. ELECTRICAL CHARACTERISTICS (Refer to Figure 1)
ABSOLUTE MAXIMUM RATINGS
(voltages referred to Pin 22) (TA = 25°C)
Parameter
Value
Units
V+ Terminal Voltage (Pin 16)
+12, -1.0
V
CS (Pin 18)
+12, -1.0
V
Speaker Amp Ground (Pin 14)
+3.0, -1.0
V
VLC (Pin 24)
VCC, -1.0
V
Storage Temperature
-65 to +150
°C
“Maximum Ratings”are those values beyong which the safety of the device cannot be
guaranteed, They are not meant to imply that the devices should be operated at these
limits. The “Electrical Characteristics” tables provide conditions for actual device
operation.
RECOMMENDED OPERATING CONDITIONS
Parameter
Value
Units
+6.0 to +11
V
CS (Pin 18)
0 to +11
V
ICC (Pin 20)
0 to 3.0
mA
VLC (Pin 24)
0.55VB to VB
V
Receive Signal (Pin 27)
0 to 250
mVrms
Microphone Signal (Pin 9)
0 to 5.0
mVrms
Speaker Amp Ground (Pin 14)
-10 to +10
mVdc
Ambient Temperature
-20 to +60
°C
V+ Terminal Voltage (Pin 16)
ELECTRICAL CHARACTERISTICS
Parameter
Symbol
Pin
V+ Supply Current
V+ = 11V, Pin 18 = 0.7V
V+ = 11V, Pin 18 = 1.6V
IV+
16
VCC Voltage ( V+ = 7.5V)
Line Regulation (6.5V< V+ <11V)
VCC
Min
Typ
Max
Units
---
---
9.0
800
mA
uA
4.9
--
5.4
65
5.9
150
Vdc
mV
SUPPLY VOLTAGES
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Output Resistance (ICC = 3.0 mA)
Dropout Voltage ( V+ = 5.0 V)
ROVCC
VCCSAT
VB Voltage (V+ = 7.5V)
Output Resistance ( IB = 1.7 mA)
---
6.0
80
20
300
ohm
mV
2.5
--
2.9
250
3.3
--
Vdc
ohm
2.0
6.0
10
dB
∆VRXO
40
-20
1.8
--
44
-16
2.3
--
48
-12
3.2
100
dB
dB
Vdc
mV
IRXOL
IRXOH
RRXI
VCR
75
1.0
3.5
24.5
--5.0
--
-3.0
8.0
32.5
uA
mA
4.0
6.0
8.0
dB
40
-16.5
1.8
--
44
-13
48
-8.5
dB
dB
2.3
--
3.2
100
Vdc
mV
--5.0
-3.0
8.0
uA
mA
----
150
6.0
75
----
mV
mV
mV
33
34
35
dB
VB
ROVB
21
ATTENUATORS
Receive Attenuator Gain (@1.0 kHz)
RX Mode, Pin 24 = VB;
Pin 27 = 250 mVrms
Range ( RX to TX Modes)
Idle Mode, Pin 27 = 250 mVrms
RXO Voltage ( RX Mode)
Delta RXO Voltage (Switch from RX to TX
Mode)
RXO Sink Current (RX Mode)
RXO Source Current (RX Mode)
RXI Input Impedance
Volume Control Range ( RX Attenuator Gain,
RX Mode, 0.6VB < Pin 24 < VB)
GRX
∆GRX
GRXI
VRXO
GTX
Transmit Attenuator Gain (@1.0 kHz)
TX Mode, Pin 3 = 250 mVrms
Range ( TX to RX Mode)
Idle Mode, Pin 3 = 250 mVrms
26,
27
3,
4
∆GTX
GTXI
TXO Voltage ( TX Mode)
Delta TXO Voltage (Switch from TX to RX
Mode)
TXO Sink Current (TX Mode)
TXO Source Current (TX Mode)
TXI Input Impedance
VTXO
∆VTXO
ITXOL
ITXOH
RTXI
∆VACF
ACF Voltage (VCC - Pin 25 Voltage)
RX Mode
Tx Mode
Idle Mode
75
1.0
3.5
20,
25
kΩ
dB
kΩ
SPEAKER AMPLIFIER
Speaker Amp Gain (Pin19 = 20mVrms)
GSPK
SKI Input Impedance
SKO Voltage ( Pin 19 = Cap Coupled to
GND)
SKO High Voltage (Pin 19 = 0.1V, -100
mA load at Pin 15)
RSKI
VSKO
15
2.4
22
3.0
37
3.6
kΩ
Vdc
VSKOH
5.5
--
--
Vdc
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SKO Low Voltage ( Pin 19 = -0.1V, +100 mA
load at Pin 15)
VSKOL
--
--
600
mV
32.5
34
35
dB
6.5
10
16
kΩ
MICROPHONE AMPLIFIER
Mike Amp Gain (Pin 9 = 10mVrms , 1.0 kHz)
GMCI
Mike Amp Input Resistance
RMCI
9,
10
LOGAMPS
RLO Leakage Current (Pin 8 = VB + 1.0V)
ILKRLO
8
--
--
2.0
uA
TLO Leakage Current (Pin 6 = VB + 1.0V)
ILKTLO
6
--
--
2.0
uA
ITH
5,7
0.8
--
1.2
--
0
--
Vdc
--
4.0
--
Vdc
Transmit
-Receive
Switching
Threshold
25
( Ratio of ITLI to IRLI - at 20 uA - to switch
TX-RX Comparator)
TRANSMIT DETECTOR
XDC Voltage -- Idle Mode
VXDC
23
-- TX Mode
CP2 Current Source
DISTORTION
ICP2
12
5.0
10
13
uA
RX Mode – RXI to SKO
RXD
27,
--
1.5
--
%
--
2.0
--
%
(Pin 27 = 10mVrms ,1.0kHz)
15
TXD
TX Mode – MCI to TXO
4,9
(Pin 9 = 5.0mVrms ,1.0kHz)
NOTES: 1.
2.
V+ = 7.5V, CS = 0.7V except where noted.
RX Mode : Pin 7 = -100 uA, Pin 5 = +100 uA, except where noted.
TX Mode : Pin 5,13
= -100 uA, Pin 7 = +100 uA, Pin 11 = 0V.
Idle Mode: Pin 5 = -100 uA, Pin 7, 13 = +100 uA.
3. Current into a pin designated as +; current out of a pin designated as - .
4. Voltages referred to Pin 22. TA = +25°C.
TEMPERATURE CHARACTERISTICS (-20 to 60°°C)
Parameter
Pin
Typical
Units
Change
V+ Supply Current (V+ = 11V, Pin 18 = 0.7V)
16
-0.2
%/°C
V+ Supply Current (V+ = 11V, Pin 18 = 1.6V)
16
-0.4
%/°C
VCC Voltage ( V+ = 7.5V)
20
+0.1
%/°C
±0.003
dB/°c
Attenuator Gain ( Max and Min Settings)
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Delta RXO, TXO Voltages
4,26
±0.24
%/°C
Speaker Amp Gain
15,19
±0.003
dB/°c
Microphone Amp Gain
9,10
±0.001
dB/°c
9
+0.4
%/°c
5,7
±0.2
nA/°c
Microphone Amp Input Resistance
Tx - Rx Switching Threshold (@ 20 uA)
6. DESIGN GUIDELINES(REF TO FIG. 1)
ATTENUATORS
The transmit and receive attenuators are complementary in function, i.e., when one is at
maximum gain the other is at maximun attenuation, and vice versa. They are never both on or both
off. Their main purpose is to control the transmit and receive paths to provide the half-duplex
operation required of a speakerphone. The attenuators are controlled safely by the voltage at the
ACF pin. The ACF voltage is provided by the Attenuator Control block, which receives 3 inputs:a)
the TX - RX Comparator, b) the Transmit Detector Comparator, and c) the Volume Control. The
response of the attenuators is based on the difference of the ACF voltage from VCC (referred to as
∆Vacf). If ∆Vacf is approximately 6mV the transmit attenuator is fully on and the receive attenuator
is fully off( transmit mode). If ∆Vacf is approximately 150mV the circuit is in the receive mode. If
∆Vacf is approximately 75mV, the circuit is in the idle mode, and the two attenuators are at gain
settings approximately half way (in dB) between their fully on and fully off positions.
The maximum gain and attenuation values are determined by the three external resistors RR,
RTX, and RRX. RR affects both attenuators according to its value RELATIVE to RTX and RRX.
RTX affects the gain and attenuation of only the transmit attenuator, while RRX affects the gain and
attenuation of only the receive attenuator. A value of 30 k is recommended for RR as a starting point,
and then RTX and RRX selected to suit the particular design goals.
The input impedance of the attenuators (at TXI and RXI) is typically 5.0 kΩ, and the
maximum input signal which will not cause output distortion is 250 mVrms(707 mVP-P). The
4300 ohm resistor and 0.01 uF capacitor at RXO (in Figure 1) filters out high frequency
components in the receive path. This helps minimize high frequency acoustic feedback problems.
The filter’s insertion loss is 1.5 dB at 1.0 kHz. The outputs of the attenuators are inverted from
their inputs.
Referring to the attenuator control block, the ∆Vacf voltage at its output is determined by
three inputs. The relationship of the inputs and ouput is summarized in the following truth table:
Volume Control
∆Vacf
Mode
Transmit
No Effect
6.0 mV
Transmit
Transmit
Idle
No Effect
75 mV
Idle
Receive
Transmit
Affects ∆Vacf
50 – 150 mV
Receive
Receive
Idle
Affects ∆Vacf
50 – 150 mV
Receive
Tx – Rx
Transmit
Comp
Det Comp
Transmit
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As can be seen from the truth table, the Tx-Rx comparator dominates. The Transmit Detector
Comparator is effective only in the transmit mode, and the Volume Control is effective only in the
receive mode.
The Tx-Rx comparator is in the transmit position when there is sufficient transmit signal
present over and above any receive signal. The Transmit Detector Comparator then determines
whether the transmit signal is a result of background noise ( a relatively stable signal) or speech
which consists of bursts. If the signal is due to background noise, the attenuators will be put into
the idle mode (∆Vacf = 75 mV). If the signal consists of speech, the attenuators will be switched
to the transmit mode (∆Vacf = 6.0 mV.) A further explanation of this function will be found in the
section on the transmit detector circuit.
The Tx-Rx comparator is in the receive position when there is sufficient receive signal to
overcome the background noise AND any speech signals. The ∆Vacf will now be 150 mV IF the
volume control is at the maximum position, i.e. VLC (Pin 24) = VB. If VLC is less than VB, the
gain of the receive attenuator, and the attenuation of the transmit attenuator, will vary in a
complementary manner. At the minimum recommended operating level (VLC = 0.55VB) the gain
of the tranmit attenuator is actually greater than that of the receive atttenuator, although it is at
receive mode. The effect of varying VLC is to vary ∆Vacf, with a resulting variation in the gains
of the attenuators.
The capacitor at ACF (Pin 25) smooths the transition between operating modes. This keeps
down any “clicks” in the speaker or transmit signal when the ACF voltage switches.
The gain separation of the two attenuators can be reduced from the typical 45 dB by adding a
resistor between Pins 20(VCC ) and 25(ACF). The effect is a reduction of the maximum ∆Vacf
voltage in the receive mode, while not affecting ∆Vacf in the transmit mode.
LOGAMPS
(Transmit and Receive Level Detectors)
The log amps monitor the levels of the transmit and receive signals, so as to tell the Tx-Rx
comparator which mode should be in effect. The input signals are applied to the amplifiers (at TLI
and RLI) through AC coupling capacitors and current limiting resistors. The value of these
components determines the sensitivity of the respective amplifiers, and has an effect on the
switching times between tranmit and receive modes. The feedback elements for the amplifiers are
back-to-back diodes which provide a logarithmic gain curve. The outputs of the amplifiers are
rectified, having a quick rise and a slow decay time. The rise time is determined primarily by the
external capacitor and an internal 500 ohm resistor, and is on the order of a fraction of 1 ms. The
decay time is determined by the external resistor and capacitor, and is on the order of a fraction
of 1 s. The switching time is not fixed, but depends on the relative values of the transmit and
receive signals, as well as these external components.
The Tx-Rx comparator responds to the voltages at TLO and RLO, which in turn are functions
of the currents sourced out of TLI and RLI, respectively. If an offset at the comparator input is
desired, e.g., to prevent noise from switching the system, or to give preference to either the
transmit or receive channel, this may be achieved by biasing the appropriate input (TLI or RLI). A
resistor to ground will cause a DC current to flow out of that input, thus forcing the output of that
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amplifier to be biased slightly higher than normal. This amplifier then becomes the preferred one
in the system operation. Resistor values from 500 k to 10 M ohms are recommended for this
purpose.
SPEAKER AMPLIFIER
The speaker amplifier has a fixed gain of 34 dB (50V/V), and is noninverting. The input
impedance is nominally 22 kΩ as long as the output signal is below that required to activate the
Peak Limiter. Since the output current capability is 100 mA, the output swing is limited to 5.0VPP
while load is 25 ohms. The output impedance depends on the output signal level and is relatively
low as long as the signal level is not near the maximum limits. At 3 VPP, it is < 0.5 ohms, and at
4.5 VPP, it is < 3 ohms. The output is short circuit protected at approximately 300 mA.
When the amplifier is overdriven, the peak limiter causes a portion of the input signal to be
shunted to ground, in order to maintain a constant output level. The effect is that of a gain
reduction caused by a reduction of the input impedance (at SKI) to a value not less than 2.0 kΩ.
The capacitor at Pin 17 (AGC) determines the response time of the peak limiter circuit.
When a large input signal is applied to SKI, the voltage at AGC will drop quickly as a current
source is applied to the external capacitor. When the large input signal is reduced, the current
source is turned off, and an internal 110 kΩ resister discharges the capacitor so the voltage at
AGC can return to its normal value (1.9 Vdc). The capacitor additionally stabilizes the peak
limiting feedback loop.
If there is a need to mute the speaker amplifier without disabling the rest of the circuit, this
may be accomplished by connecting a resistor from the AGC pin to ground. A 100 kΩ resister
will reduce the gain by 34 dB (0 dB from SKI to SKO), and a 10 kΩ resister will reduce the
gain by almost 50 dB.
TRANSMIT DETECTOR CIRCUIT
The transmit detector circuit, also known as the background noise monitor, distinguishes
speech (which consists of bursts) from the background noise ( a relatively constant signal). It does
this by storing a voltage level, representative of the average background noise, in the capacitor at
CP1( Pin 11). The resistor and capacitor at this pin have a time constant of approximately 5
seconds(in Figure 1). The voltage at Pin 11 is applied to the inverting input of the Transmit
Detector Comparator. In the absence of speech signals, the noninverting input receives the same
voltage level minus an offset of 36 mV. In this condition, the output of the comparator will be low,
the output transistor turned off, and the voltage at XDC (Pin 23) will be at ground. If the Tx-Rx
comparator is in the transmit position, the attenuators will be in the idle mode ( ∆Vacf = 75 mV).
When speech is presented to the microphone, the signal burst appearing at XDI reaches the
noninverting input of the transmit detector comparator before the voltage at the inverting input
can change, causing the output to switch high, driving the voltage at XDC up to approximately 4
V. This high level causes the attenuator control block to switch the attenuators from the idle mode
to the transmit mode (assuming the Tx-Rx comparator is in the transmit mode).
The series resistor and capacitor at XDI (Pin 13) determine the sensitivity of the transmit
detector circuit. Increasing the resistor, or lowering the capacitor, will reduce the response at CP2
and CP1.
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The response at CP2, CP1, XDC to a varying signal at the microphone is as follows:
1. CP2 ( Pin 12) follows the peaks of the speech signals, and decays at a rate determined by the
10 uA current source and the capacitor at CP2.
2. CP1 (Pin 11) increases at a rate determined by the RC at this pin after CP2 has made a positive
transition. It will follow the decay pattern of CP2.
3. The noninverting input of the Transmit Detector Comparator follows CP2, gained up by 2.7,
and reduced by an offset 36 mV. This voltage, compared to CP1, determines the output of the
comparator.
4. XDC (Pin 23) will rise quickly to 4 Vdc in response to a positive transition at CP2, but will
decay at a rate determined by the RC at this pin. When XDC is above 3.25 Vdc, the circuit will
be in the transmit mode. As it decays towards ground, the attenuators are taken to idle mode.
MICROPHONE AMPLIFIER
The microphone amplifier is noninverting, has an internal gain of 34 dB(50 V/V), and a
nominal input impedance of 10 kΩ. The output impedance is typically < 15 ohms. The maximum
p-p voltage swing available at the output is approximately 2.0 V less than VCC, which is
substantially more than what is required in most applications. The input at MCI(Pin 9) should be
ac coupled to the microphone so as to not upset the bias voltage. Generally, microphone sensitivity
may be adjusted by varying the 2 k microphone bias resistor, rather than by attempting to varying
the gain of the amplifier.
POWER SUPPLY
The voltage supply at V+ (Pin 16) should be in the range of 6.0 to 11 V, although the circuit
will operate down to 4.0 V. The voltage can be supplied either from Tip and Ring, or from a
separate supply.
It is imperative that the V+ supply be a good ac ground for stability reasons. If it is not well
filtered (by a 1000 uF capacitor AT THE IC), any variation at V+ caused by the required speaker
current flowing through this pin can cause a low frequency oscillation. The result is usually that
the circuit will cut the speaker signal on and off at the rate of a few hertz.
VCC
VCC (Pin 22) is a regulated output voltage of 5.4 ±0.5 V. Regulation will be maintained as
long as V+ is (typically) 80 mV greater than the regulated value of VCC. Up to 3 mA can be
sourced from this supply for external use. The output impedance is < 20 ohms. The 47 uF
capacitor connecting to this pin is essential for stability reasons. The
capacitor must be located adjacent to the IC.If the circuit is deselected, the VCC will go to 0 V.
VB
VB is a regulated output voltage with a nominal value of 2.9±0.4 V. It is derived from VCC
and tracks it, holding a value of approximately 54% of VCC. 1.5 mA can be sourced from this
supply at a typically output impedance of 250 ohms. The 47 uF capacitor connecting to
this pin is essential for stability reasons. The capacitor must be ocated
adjacent to the IC. If the circuit is deselected, the VB will go to 0 V.
CHIP SELECT
The CS pin (Pin 18) allows the chip to be powered down anytime its functions are not
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required. A logic “1” level in the range of 1.6 V to 11 V deselects the chip. The input resistance at
Pin 18 is > 75 kΩ. The VCC and VB regulated voltages go to 0.0 when the chip is deselected.
Leaving Pin 18 open is equivalent to a Logic “0” (chip enabled).
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SWITCHING TIME
The switching times of the speakerphone circuit depends not only on the various external
components, but also on the operating conditions of the circuit at the time a change is to take
effect. For example, the switching time from idle to transmit is generally quicker than the
switching time from receive to transmit ( or transmit to receive).
The components which most significantly affect the timing between the transmit and receive
modes are those at Pins 5 (transmit turn-on), 6 (transmit turn-off), 7 (receive turn-on), and 8
(receive turn-off). These four timing functions are not independent, but interactive since the Tx-Rx
comparator operates on a RELATIVE Tx-Rx comparison, rather than on absolute values. The
components at Pins 11, 12, 13 and 23 affect the timing from the transmit mode to the idle mode.
Timing from the idle mode to transmit mode is relatively quick ( due to the quick charging of the
various capacitors), and is not greatly affected by the component values. Pins 5-8 do not affect the
idle-to-transmit timing since the Tx-Rx comparator must already be in the transmit mode for this
to occur.
The following table provides a summary of the effect on the switching time of the various
components, including the volume control:
Components
Tx to Rx
Rx to Tx
Tx to Idle
RC @ Pin 5
Moderate
Significant
No Effect
RC @ Pin 6
Significant
Moderate
No Effect
RC @ Pin 7
Significant
Moderate
No Effect
RC @ Pin 8
Moderate
Significant
No Effect
RC @ Pin 11
No Effect
Slight
Moderate
No Effect
Slight
Significant
RC @ Pin 13
No Effect
Slight
Slight
RC @ Pin 23
No Effect
Slight
Significant
[email protected] Pin 24
No Effect
Moderate
No Effect
[email protected] Pin 25
Moderate
Moderate
Slight
C @ Pin 12
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BL34018
Additionally, the following should be noted:
1) The RCs at Pins 5 and 7 have a dual function in that they affect the sensitivity of the respective log
amplifiers, or in other words, how loud the speech must be in
order to get control of the speakerphone
circuit.
2) The RC at Pin 13 also has a dual function in that it determines the sensitivity of
the transmit detector
circuit.
3) The volume control affects the switching speed, and the relative response to transmit signals, in the
following manner: When the circuit is in the
increases the signal at
receive mode, reducing the volume control setting
TXO, and consequently the signal to the TLI pin. Therefore a given
at TXI will switch the circuit into the transmit mode quicker at
signal
low volume settings.
APPLICATIONS INFORMATION
The BL34018 speakerphone IC is designed to provide the function additionally required
when a speakerphone is added to a standard telephone. The IC provides the necessary relative
level detection and comparison of the speech signals provided by the talkers at the
speakerphone(near end speaker) and at the distant telephone(far end speaker).
The BL34018 is designed for use with an electorate type microphone a 25 ohms speaker, and
has an output power capability of 100mW.All external components surrounding this device are
passive, however, this IC does require additional circuitry to interface the TIp and Ring telephone
lines. Two suggested circuits are shown in the data sheet.
Fig 16 depicts a configuration which does not include a handset, dialer, or ringer. The only
controls are S1(to make the connection to the line),S2(a "privacy" switch),and the volume control.
It is meant to be used in parallel with a normal telephone which has dialing and ringing functions.
Fig 17 depicts a means of providing logic level signal that indicate which mode of operation
the BL34018 is in. Comparator A indicates whether the circuit is in receive or transmit mode and
comparator B indicates(when in transmit/idle mode) whether the circuit is in the transmit or idle
mode. The LM393 dual comparator is chosen because of its low current requirement (<1.0mA),
low voltage requirement (as low as 2.0volts), and low cost .
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- 15 Total 16 Pages
8/15/2006
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BL34018
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- 16 Total 16 Pages
8/15/2006
Wrote by 2006