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PBL385411SO-T Datasheet(PDF) 11 Page - Ericsson |
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PBL385411SO-T Datasheet(HTML) 11 Page - Ericsson |
11 / 14 page 11 PBL 385 41 Power supplies DC1, DC2, V+C and VPA (See fig.18) PBL 38541 generates its own DC supply V+C dependent of line current with an internal shunt regulator. This regulator senses the line voltage VL via R3 and line current via R6 in order to set the correct V+C so the circuit can generate the required DC characteristic for a given line resistance RLine and the line feeding data of the exchange. A decoupling capacitor is needed between pins +C and -L. The V+C supply changes its voltage linearly with the line current. It can be used to feed an electret microphone. Caution must be ta- ken though not to drain too much current out of this output because it will affect the + RFeed RLine IL 1 Hook switch ƒ Lim + V Tx + - 3 2 4 +Line + + -Line C1 R3 R6 VMon. + 4.7-47 µF 0-470 Ω 1-10M VDC2 Ref. 1.2V VDC1 + 4.7-47 µF VPA + a. b. VL Vexh. a. Supply arrangement for a handsfree system power amplifier. For ex. PBL 388 13 b. Supply arrangement for a call monitor cicuit. V+C + - 15k 15k 8 9 7 14 6V 6V PBL 385 41 R17 3.7V internal quick start circuit by locking itself into active state. (max. permissible current drain 700 µA) Care has to be taken when deci- ding the resistance value of R3. All resistances that are applied from +Line to ground (-Line) will be in parallel, forming the real impedance towards the line. This will sometimes result in, that the ohmic value of R3 is increased in order to comply to the impedance specification towards the line. The speech circuit sinks ≈ 1mA into pin 4, which means that the working voltage for the speech function V+ will decrease with increasing R3, thus starving in the end the circuit of its working voltage . This dependency is often falsely taken as a sign of that the circuit does not work down to the low line current specified, but in fact it is the working voltage at pin 4 that has become too low. It is obvious that this problem is also connected into what kind of DC-characteristic is set. See fig. 8. The circuit has further two tempera- ture and line current compensated DC supplies DC1 and DC2. DC2 is a voltage supply for supplying diallers, can be used for memory back up because it does not leak any current back into the circuit. Typical voltage 2.4V down to line voltage of 4.1V, in case the line voltage is lower than 4.1V calculate ; actual line voltage minus 1.9V. In order to prevent noise entering the line, a series resistor and a reservoir capaciotor is recommended in for this output.The output current is given to be 2 mA in the specification. DC1 is a 3.7 V (typ.) supply whose, for the design required maximum output current can be set at the control input pin 7. The set current will flow constant (necessary to keep the line current cons- tant) used or not used by a function, so care has to be taken when setting the current limit so it won´t be unnecessary high. The maximum current that can be set and drawn by maintaining the voltage level is 2.0 mA (up to 6 mA can be set if the line current so allows). This supply is ment to be used to supply microphones, Ericsson´s handsfree IC’s switching parts (see applications), opto couplers etc. and in payphones its auxiliary functions. The fourth DC-supply VPA has an advantage that it does not influence the circuits DC characteristics even at high current drain. The supply has a floating ground reference in the +line in order to minimize RFI problems and is used to supply the power amplifier of a handsfree telephone ( PBL3881, 38813 ). These circuits have a current controlled charging of the supply capacitor and the control signal is taken across the resistor R6. In case a monitor amplifier is required where the ground reference is hardly necessary, it can be supplied from VPA or like in alt. b in fig. 18. Figure 18. DC - supply system for external loads. DTMF input. The circuit provides a specific input at pin 10 for DTMF dialling signals. The output of the amplifier is controlled by the mute signal provided by the dialler. This input makes it easy to time the DTMF signals entering the line. Most of the diallers wake up at pressing of a key and the output will not be stable imidiately. This unstable state will be of some length and is limited in some telephone specifications. |
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