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UBA1707

Cordless telephone, answering machine line interface

Philips Electronics 

Philips Semiconductors

Cordless telephone, answering machine

line interface

Product specification

UBA1707

The DC line current flowing into the set is determined by

the exchange supply voltage (Vexch), the feeding bridge

resistance (Rexch), the DC resistors of the telephone line

(Rline) and the set (RSET), the reference voltage (Vref) and

the voltage introduced by the transistor (TNSW) used as

line interrupter (see Fig.5). With a line current below Ilow

(8 mA with ZSET = 619 Ω), the internal reference voltage

(Vref) is automatically adjusted to a lower value. This

means that more sets can operate in parallel with DC line

voltages (excluding the polarity guard) down to 1.2 V.

At line current below Ilow, the circuit has limited transmit

and receive levels. This is called the low voltage area.

Figure 6 shows in more details how the UBA1707, in

association with some external components, manages the

line interrupter (TNSW external transistor).

In on-hook conditions (voltage at pin EHI is LOW), the

voltage at pin LCC is pulled-up to the supply voltage level

(VCC) to turn off the TPDARL transistor. As a result, because

of the RPLD resistor, the TNSW and TNON-HOOK transistors

are switched off. The TNON-HOOK transistor disconnects

the RLVI resistor from the LN− line terminal in order to

guarantee a high on-hook impedance.

In off-hook conditions (voltage at pin EHI is HIGH), an

operational amplifier drives (at pin LCC) the base of

TPDARL which forms a current amplifier structure in

association with TNSW. The line current flows through

TNSW transistor. The TNON-HOOK transistor is forced into

deep saturation. A virtual ground is created at pin LVI

because of the operational amplifier. A DC current (ILVI) is

sourced from pin LVI into the RLVI resistor in order to

generate a voltage source. Thus the voltage between pin

GND and the negative line terminal (LN−) becomes:

VCE (TNSW) = RLVI × ILVI + VCE (TNON-HOOK) ≅ RLVI × ILVI

The voltage Vline between the line terminals LN+ and LN−

can be calculated as follows:

Vline ≅ Vref + RSLPE × (Iline − IZSET) + VCE (TNSW)

Where:

Iline = line current

IZSET = current flowing through ZSET.

1999 Feb 17

8

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