BL34018
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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Total 16 Pages
Wrote by 2006
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