TCA3388

11

MOTOROLA ANALOG IC DEVICE DATA

factor m. The relation between the line current and the factor

m is depicted in Figure 11.

II

I

0

0.5

1.0

m

I

Figure 11. Modulation Factor m as a Function

of Line Current

Line

Lstop

Lm

Lstart

For low line currents below ILstart, thus long lines, the

factor m equals 1. This means the hybrid network ZHL is fully

used. For high line currents above ILstop, thus short lines, the

factor m equals 0. This means the hybrid network ZHS is fully

used. Both networks are used 50% for the intermediate line

current Ilm.

The switch over between the 2 networks takes place in

region 3 for the French and U.K. mask and in region 2 for the

Low Voltage mask.

LINE LENGTH AGC

The TCA3388 offers the possibility to vary the transmit and

receive gain over line length in order to compensate for the

loss in gain at longer line lengths. In the block diagrams of the

transmit and receive channels (Figures 9, 10) the line AGC is

drawn. The line AGC can be switched off by connecting a

150 k

Ω resistor between HSO and Gnd. In this case, the

transmit and receive gain are lowered by 2.0 dB with respect

to the value calculated in the formulas above.

The line AGC characteristics for both transmit and receive

channel have the general shape depicted in Figure 12.

I

I

Reduced

Gain

Nominal

Gain

Gain

I

Figure 12. General Line AGC Characteristics

Line

Lrange

Lstart

Gain

+

Nominal Gain

1

)

I

L

–I

Lstart

I

Lrange

For low line currents, and thus long lines, the gains are

nominal. When the line current has increased above ILstart

with a current ILrange, the gain is reduced by 6.0 dB. Due to

the general characteristics of the line AGC curve, the gain will

be decreased further for higher currents.

For France and U.K., the line AGC will be active in region

3 of the dc characteristics. The ILstart is approximately equal

to the ILK. The range is calculated from:

I

Lrange +

Z1 x (I2R – I2CD)

R

E3

For Low Voltage mask, the line AGC is active in region 2.

DIALING

Pulse–dialing is performed by making pin PI high. As a

result the output LAO goes low and the loop will be

disconnected. Internally the current consumption of the

circuit is reduced and the current through the microphone is

switched off.

DTMF–dialing is performed by supplying a DTMF signal

current to Pin LAI. This is the same node where the

microphone signal currents are internally applied. Therefore,

for the DTMF gain the same formulas apply. Because the

microphone preamplifier is bypassed, there is no influence

on DTMF signals by the line length AGC.

A DTMF confidence tone can be generated on the

earpiece by injecting a signal current at the RXI pin. Because

only the earpiece amplifier itself is used, there are no effects

from AGC or hybrid switchover.

For correct DTMF–dialing the pin MUT has to be made

high. This mutes both the microphone and earphone

preamplifier. In this way signals from the microphone will not

be amplified to the line and signals from the line are not

amplified to the earpiece.

The complete interfacing of the DTMF generator with the

TCA3388 is shown in the typical application.

SUPPORT MATERIAL

Device Specification:

Brief description of the TCA3388,

block diagram, device data, test

diagram, typical application

User manual TCA3388: Extended description of the circuit

and its concept, adjustment

procedure, application hints and

proposals

Demonstration board:

Shows performance of the TCA3388

in its basic application

TYPICAL APPLICATION

The typical application below is based on the demoboard

of the TCA3388. It contains the speech transmission part,

diode bridge, hook switch and microcontroller interfacing.

The dc mask setting on the bottom left is given for France,

U.K. and Low Voltage applications. The component values

are given in the table of Figure 14. The line driver is extended

with T1, D5 and R3 which increases the signal swing under

low line voltage conditions.