COUNTER FEEDING

It is possible that, in some applications, a commu-

nication channel that the STLC5444 feeds, is also

biased at the other end by another feeding de-

vice.

What considerations apply to the STLC5444 in

this case?

Let’s use a generic example for reference (see Fig. 3)

STLC5444

Si

V

BB

U

S

STLC5444

V

BB

Si

PABX

NT

S

S

D95TL229

Figure 3: Typical PABX connection.

A PABX with S-interfaces may have some of

them connected to Terminal Equipments, and one

to the S-interface of a Nertwork Termination. The

S-interface of the PABX connected to the NT has

one channel of the STLC5444 available for feed-

ing. It will be programmed in the ”OFF” state to

avoid interference with the feeding coming from

the NT (of course the feeding coming from the NT

will not be loaded by this PABX connection).

The following considerations are relevant in the

above example:

1) The VBB of the STLC5444 in the PABX must

be equal or more negative than the feeding

voltage coming from the NT (unless decou-

pling diodes are externally provided - see 4)

2) The STLC5444 channel of the PABX must be

programmed OFF.

3) In the channel of the STLC5444 on the PABX

side, the only effect will be on the relevant

LVD bit that will be set to 1 if the feeding volt-

age coming from the NT is 3V more positive

than the local VBB. No interrupts or alarms are

generated.

4) It is good common practice to provide every S-

interface with protection circuitry against tran-

sient overvoltages (see Fig. 4). This includes a

diode in series with each Si pin of the

STLC5444. If this is the case, absolute levels

of local VBB and NT feeding are no concern at

all. (If such diodes are not present, care must

be paid to the power supply of the PABX, and

to the connected circuits. When the PABX

supply is OFF, the NT feeding will find a con-

nection through the relevant channel of the

STLC5444 to the VBB point).

TPAXX

D1

D2

D3

Si

BGND

BGND

STLC5444

FUSE RESISTORS

S INTERFACE

BATTERY

D95TL230

Figure 4: Protection of the STLC5444 against overvoltage.

STLC5444

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