L3037 datasheet(6/22 Pages) STMICROELECTRONICS

6 / 22 page

the ext. transistor to a proper value higher than

the real negative battery in order to reduce the

power dissipated by the L3037 itself.

b) Resistive feed region: the characteristic is

equal to a battery voltage (Vbat) in series with a

resistor (typ 400ohm or 800ohm) whose value is

set by one ext. resistor (see ext. components list).

c) Constant voltage region: the characteristic is

equal to the battery voltage - 12V in series with

the ext. protection resistors (typ 80ohm).

This voltage drop between battery and line termi-

nals for Il=0 allows on-hook transmission.

Fig. 1 shows the DC characteristic in active

mode. Fig. 2 shows the line current versus loop

resistance

In active mode the AC impedance at the line ter-

minals is sinthetized by the external components

ZAC and Rp according to the following formula:

Zs = ZAC/50 + 2*Rp

Depending on the characteristic of the ZAC net-

work Zs can be either a pure resistance or a com-

plex impedance. This allows L3037 to meet diffe-

rent standard as far as return loss is concerned.

The capacitor CCOMP guarantees stability to the

system.

The two to four wire conversion is achieved by

means of a circuit that can be represented as a

Wheastone bridge, the branches of which are:

1) The line impedance (Zline)

2) The SLIC impedance at line terminals (Zs)

3) The balancing network ZA+RA connected

between pin ZAC and ZB of L3037.

4) The network ZB between pin ZB and GND

that shall copy the line

impedance.

When L3037 is used with a second generation

combo (eg TS5070FN) which is able to perform the

two to four wire conversion, the two impedances

ZA and ZB can be removed and the ZB pin con-

nected to GND. The -6dB TX gain of the L3037 al-

lows the echo signal to remain always within the

COMBOII Hybrid balance filter dynamic range.

The injection of high frequency metering pulses is

carried out through the SLIC. An unbalanced 12 or

16KHz sinusoidal signal with shaping is, when ne-

cessary, applied at the TTXIN input of the SLIC.

A fixed transfer gain is provided for the metering si-

gnal. To avoid saturation in the 4-wire side a can-

cellation is provided in the 4-wire transmission path.

Cancellation is obtained via an external RC net-

work without the need for trimmed components.

When the TTX function is not used TTXIN input

should be connected to GND. Since this pin is di-

rectly connected to a summing node inside the

SLIC any signal applied to the TTXIN is transfer-

red to the line with a fixed transfer gain.

In special applications, this pin can be used to

modify the voltage drop (constant voltage region

of DC characteristic) simply by applying a proper

DC level on the TTXIN pin, allowing optimization

of the battery voltage versus the maximum nee-

ded AC signal swing.

In active mode, with a -48V battery voltage, the

L3037 dissipate 150mW for its own operation (in-

cluding the power dissipation from +5/-5 supply),

the dissipation related to the current supplied to

the line should be added in order to get the total

dissipation.

STAND-BY MODE (SBY)

In this mode the bias current of the L3037 is redu-

ced and only some part of the circuit are completely

active. The transversal current supplied to the line

is limited at 14mA. Common mode current rejection

is performed and the total current capability of the

output stages (TIP and RING) is limited to 30mA.

The open circuit voltage is |Vbat|-7V.

Both Off/Hook and Ground key detectors are acti-

ve. Signal transmission is not operating.

In stand-by mode, with a -48V battery voltage, the

L3037dissipates90mW typ. (including the power dis-

sipationfrom a +5/-5V supply).

Stand-by mode is usually selected when the telepho-

ne is in on-hook condition. It allows a proper off-hook

detection, even in the presence of high common

mode currents, or with telephone sets sinking a few

milliamperes of line currentin on-hook condition.

Figure 1: DC Characteristic in active mode

Figure 2: Current vs. Loop Resistance.

Rfeed = 2 x 200ohm,

Lim. currents: 25, 43, 55mA

L3037

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