TCA3388

8

MOTOROLA ANALOG IC DEVICE DATA

The TCA3388 offers the possibility to connect 2 sidetone

networks Z1 and Z2. For correct dc operation, the dc

impedance of these networks must be equal. When only 1

sidetone network is used, Pin HYS has to be connected to

HYL. All formulas below are based on a single sidetone

network, so only Z1 appears. When 2 sidetone networks are

used, Z1 has to be replaced by Z1//Z2.

In region 1, the transfer of the amplifier G at the HYL/HYS

inputs equals zero. The voltage difference between SAO and

SAI will equal VO1. The slope RE1 of the VLine, ILine

characteristic will equal:

R

E1 +

R1 x

1

) Z0

Z21

In region 2, the output current of the amplifier G will be

proportional to the input current. As a result the voltage

between SAO and SAI will increase with the line voltage.

Speech signals on the line are of no influence on this

because they are filtered out via capacitor C16. The slope

RE2 of the VLine, ILine characteristic will equal:

R

E2 +

R1 x

1

)

1

RI

Z1

) Z21

Z0

In region 3, the output current of the amplifier G is kept

constant. As a result the slope in region 3 will equal the slope

of region 1.

The transfer from region 2 to 3 occurs at the point VLK, ILK

defined by:

I

LK +

Z21

Z0

x Z1 x I2CD

) 2V

BE )

V

CD

R1

VLK = Z1 x I2CD + 2 VBE + VCD + VO2

With:

and 2 VBE  1.4 V, V02  1.1 V

I2CD

+ I2C ) I2D

2

,and V

CD +

V

C )

V

D

2

,

When the French or U.K. mask is selected, this transfer

takes places for line currents of 30 mA to 40 mA depending

on the components settings. With the Startup and Low

Voltage mask, the transfer lies outside the normal operating

range with line currents of 90 mA or more. In most

applications the transfer from region 1 to 2 takes place for line

currents below 10 mA.

With proper settings, region 4 is entered only during an

overload condition. In this mode, the power consumption in

the telephone set is limited. In order to detect an overload

condition, the voltage between the Pins LAI and SAO is

monitored. When the voltage difference is larger than the

threshold VClamp1, the protection is made active. The relation

for the line voltage VLP at this point is given as:

V

LP +

Z0

Z21

xV

Clamp1 )

V

CD )

VO2

When the protection mode is entered, the line current is

reduced to a lower value ILP of:

I

LP +

V

Clamp2 )

(VO1 – VO2)

R1

When the line voltage becomes lower than VLP, the

overload condition is removed and the TCA3388 will leave

region 4.

The current drawn from the line by the dc part is used to

supply the TCA3388 and peripheral circuits. The excess loop

current is absorbed by the voltage regulator at Pin VCC,

where a filter capacitor is connected. The reference for the

circuit is Pin Gnd.

Startup of the application is ensured by an internal startup

circuit. When the line current flows, the hook status output

pin HSO goes high. This informs the microcontroller that the

set is off–hook. When the line current is no longer present the

pin will go low again. Because the line current is monitored,

and not the line voltage, also an interrupt of the exchange

can be recognized.

AC CHARACTERISTICS

Impedance

In Figure 6, the block diagram of the TCA3388 performing

the ac impedance is depicted. As can be seen it is partly

common with the dc mask block diagram. The part

generating the dc mask is replaced by a dc voltage source

because for ac, this part has no influence.

Line +

Line –

Figure 6. AC Stage of the TCA3388

T3

T2

Z0

R5

V02

C7

V

LAI

LAO

Gnd

R1

TCA3388

CC

Z21

SAO

SAI

VO1

When calculating the ac loop, it can be derived that the set

impedance Zin equals

Z

in +

V

Line

I

Line

+ R1 1 ) Z0

Z21

[ R1 x Z0

Z21

As can be noticed, the formula for the ac impedance Zin

equals the formula for the dc slope in regions 1 and 3.

However, because for the dc slope the resistive part of Z0

and Z21 are used, the actual values for Zin and the dc slopes

do not have to be equal.

A complex impedance can be made by making either Z0

or Z21 complex. When Z0 is made complex to fit the set

impedance the transmit characteristics will be complex as

well. The complex impedance is therefore preferably made

via the Z21 network. Because Z21 is in the denominator of

the Zin formula, Z21 will not be a direct copy of the required

impedance but a derivative of it. Figure 7 gives this derived

network to be used for Z21.