MT91600
Loop Supervision & Dial Pulse Detection
Data Sheet
The Loop Supervision circuit monitors the state of the phone line and when the phone goes "Off Hook" the SHK pin
goes high to indicate this state. This pin reverts to a low state when the phone goes back "On Hook" or if the loop
resistance is too high for the circuit to continue to support a constant current.
The SHK output can also be monitored for dialing information when used in a dial pulse system.
Constant Current Control
The SLIC employs a feedback circuit to supply a constant feed current to the line. This is done by sensing the sum
of the voltages across the feed resistors, R1 and R2, and comparing it to the input reference voltage, Vref, that
determines the constant current feed current.
The MT91600’s programmable current range is between 18 mA to 32 mA.
Line Drivers & Overcurrent Protection
The Line Drivers control the external Battery Feed circuit which provide power to the line and allows bi-directional
audio transmission.
The loop supervision circuitry provides bias to the line drivers to feed a constant current while the over-current
protection circuitry prevents the ring driver from causing the ring transistor to overload.
The line impedance presented by the Line Driver circuitry is determined by the external network, which may be
purely resistive or complex, allowing the circuit to be configured for use in any application. The impedance can also
be fixed to one value and modified to look like a different value by reflecting an impedance through the SLIC from
an intelligent CODEC or DSP module.
There is long term protection on the RING output against accidental short circuits that may be applied either across
TIP/RING to GND or RING to GND. This high current will be sensed and limited to a value that will protect the
circuit.
In situations where an accidental short circuit occurs either across TIP/RING to GND or RING to GND, an
excessive amount of current will flow through the ring drive transistor, Q3. Although the MT91600 will sense this
high current and limit it, if the power rating of Q3 is not high enough, it may suffer permanent damage. In this case,
a power sharing resistor, R23, can be inserted (see Figure 5) to dissipate some of the power. Capacitor C13 is
inserted to provide an a.c. ground point. The criteria for selecting a value for the power sharing resistor R23 can be
found in the application section of this data sheet.
Ringing and Ring Trip Detection
Ringing is applied to the line by disconnecting pin 8, RF, from pin 9, RV, and connecting it to a ringing source which
is battery backed. This may be done by use of an electro-mechanical relay. The SLIC is capable of detecting an Off
Hook condition during ringing by filtering out the large A.C. component by use of the external components
connected to pin 23. This filter allows an Off Hook condition to be monitored at SHK, pin 24.
When using DTMF signalling only i.e., pulse dialling is not used, the capacitor, C7, can be permanently connected
to ground and does not require to be switched out during dialling.
Power up Sequence
The circuit should be powered up in the following order: AGND, VEE, VDD, V
BAT.
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Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
