TP5700AN datasheet(6/8 Pages) NSC

Il numero della parte	TP5700AN

Spiegazioni elettronici	Telephone Speech Circuit

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Produttore elettronici	NSC [National Semiconductor (TI)]

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Applications Information

The TP5700A and TP5700 are flexible circuits designed

with several user adjustments to enable the performance to

be optimized for different applications The choice of trans-

ducer types and the cavities in which they are mounted will

also greatly influence the acoustic performance of the tele-

phone Some of the consequences of circuit adjustments

are as follows

RDC ADJUSTMENT

56X is the recommended value for RDC if it is required to

meet a maximum Tip-Ring voltage of 45V on a 20 mA loop

(assuming no more than 14V is dropped across the polarity

guard) If a higher Tip-Ring voltage is acceptable RDC may

be increased which will provide a small increase in the

available transmit output voltage swing before clipping oc-

curs RDC should be less than 82X to avoid exceeding the

maximum rated voltage on a short loop

RAGC ADJUSTMENT

The available AGC range is more than adequate to compen-

sate for the loss of most loops RAGC should be chosen

only to partly compensate for the anticipated maximum loop

loss as over-compensation may tend to exaggerate the var-

iations of sidetone with loop length

SIDETONE ADJUSTMENT

The component values used for ZBAL should be selected to

provide a clear sidetone sound without excessive ‘‘hollow-

ness’’ The capacitor value and ratio of the two resistors will

fix the pole location To avoid reducing the low voltage per-

formance of the circuit the sum of the two resistors should

not exceed 10 kX

POWERING ELECTRET MICROPHONES

Electret microphones with integral FET buffers are available

in both two-wire and three-wire versions and a range of op-

erating voltage ranges There are four methods of powering

the microphone

1 The 12V VREG2 output provides the lowest voltage meth-

od for microphones rated down to 1V VREG2 must be

decoupled with a 1 mF capacitor to ground (See

Figure

5 )

2 If VREG1 is not required for DTMF generator operation it

may be used to provide up to 200 mA for microphone

power

3 VBIAS (pin 15) may be used as a decoupled but unregu-

lated supply for electrets requiring a higher operating

voltage than VREG1 or VREG2 The additional current

drawn through Ro will however raise the minimum oper-

ating voltage of the Speech Circuit If this method is used

the decoupling capacitor must be increased to at least

100 mF to maintain good low frequency return loss (See

Figure 4 )

4 An electret type with a good power supply rejection ratio

can be powered from Va or a regulated and decoupled

supply dropped from Va

TONE DIALING TELEPHONE

Figure 4 shows the TP5700 directly interfacing to a low volt-

age DTMF generator VREG1 supplies the necessary 2V

minimum bias to enable the low voltage tone dialer to sense

key closures and pull its MUTE output high VREG1 then

switches to a 3V regulated output to sustain the Tone Dialer

during tone generation The TP5700A DTMF input incorpo-

rates the necessary load resistor to Vb and provides gain

plus AGC action to compensate for loop length A muted

tone level is heard in the receiver For DTMF generators

with a higher output level a resistive potentiometer should

TLH5201 – 8

FIGURE 4 Typical Tone Dialing Telephone

6

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