MITEL MT91600 Datasheet
MT91600
Programmable SLIC
Preliminary Information
Features
• Transformerless 2W to 4W conversion
• Controls battery feed to line
• Programmable line impedance
• Programmable network balance impedance
• Off-hook and dial pulse detection
• Ring ground over-current protection
• Programmable gain
• Programmable constant current feed
• -22V to -72V battery operation
Applications
Line interface for:
• PABX/ONS
• Intercoms
• Key Telephone Systems
• Control Systems
DS5057 ISSUE 7 August 1999
Package Information
MT91600 28 Pin SSOP Package
-40°C to +85°C
Description
The Mitel MT91600 provides an interface between a
switching system and a subscriber loop, mainly for
short loop SLIC applications. The functions provided
by the MT91600 include battery feed, programmable
constant current, 2W to 4W conversion, off-hook and
dial pulse detection, user definable line and network
balance impedance’s and the capability of
programming the audio gain externally. The device is
fabricated as a CMOS circuit in a 28 pin SSOP
package.
TD
TF
TIP
RING
RF
C3A
C3B
RV
RD
Tip Drive
Controller
Line Sense
Over-Current
Protection Circuit
Ring Drive
Controller
X3 X2 X1
Audio Gain & Network
Balance Circuit
2 W to 4 W
Conversion & Line
Impedance
Loop Supervision
Figure 1 - Functional Block Diagram
Relay
Driver
VX
VR
Z3
Z2
Z1
RLYC
RLYD
VEEGNDVDDC2BC2AC1SHKVREFIC
1
MT91600 Preliminary Information
VDD
TD
TF
TIP
RING
VREF
RF
RV
RD
C3A
C3B
C2B
C2A
1
2
3
4
5
6
7
IC
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
VEE
GND
RLYD
RLYC
SHK
C1
X2
VR
X3
VX
X1
Z3
Z2
Z1
Figure 2 - Pin Connections
Pin Description
Pin # Name Description
1 VDD Positive supply rail, +5V.
2TDTip Drive (Output). Controls the Tip transistor.
3TFTip Feed. Connects to the Tip transistor and to the TIP lead via the Tip feed resistor.
4 TIP Tip. Connects to the TIP lead of the telephone line.
5 RING Ring. Connects to the RING lead of the telephone line.
6 VREF Reference Voltage (Input). This pin is used to set the subscribers loop constant
current. Changing the input voltage sets the current to any desired value within the
working limits. VREF is related to VLC.
7ICInternal Connection (Input). This pin must be connected to GND for normal operation.
8RFRing Feed. Connects to the RING lead via the Ring feed resistor.
9RVRing Voltage and Audio Feed.Connects directly to the Ring drive transistor and also to
Ring Feed via a relay.
10 RD Ring Drive (Output). Controls the Ring transistor.
11 C3A A filter capacitor for over-current protection is connected between this pin and GND.
12 C3B A filter capacitor for over-current protection is connected between this pin and GND.
13 C2B A capacitor for loop current stability is connected between this pin and C2A.
14 C2A A capacitor for loop current stability is connected between this pin and C2B.
15 Z1 Line Impedance Node 1. A resistor of scaled value "k" is connected between Z1 and
Z2. This connection can not be left open circuit.
16 Z2 Line Impedance Node 2. This is the common connection node between Z1 and Z3.
17 Z3 Line Impedance Node 3. A network either resistive or complex of scaled value "k" is
connected between Z3 and Z2. This connection can not be left open circuit.
18 X1 Gain Node 1. This is the common node between Z3 and VX where resistors are
connected to set the 2W to 4W gain.
19 VX Transmit Audio (Output). This is the 4W analog signal to the SLIC.
20 X3 Gain Node 3. This is the common node between VR and the audio input from the
CODEC or switching network where resistors are fitted to sets the 4W to 2W gain
21 VR Receive Audio (Input). This is the 4W analog signal to the SLIC.
2
Preliminary Information MT91600
Pin Description (continued)
Pin # Name Description
22 X2 Gain Node 2. Networks, either resistive or complex, are connected between this node,
VR and GND to set the Network Balance Impedance for the SLIC.
23 C1 A filter capacitor for ring trip is connected between this pin and GND.
24 SHK Switch Hook (Output). This pin indicates the line state of the subscribers telephone.
The output can also be used for dial pulse monitoring. SHK is high in off-hook state.
25 RLYC Relay Control (Input). An active high on this pin will switch RLYD low.
26 RLYD Inverted Output of RLYC. It is used to drive the bipolar transistor that drives the relay
(see Figure 5.)
27 GND Ground. Return path for +5V and -5V. This should also be connected back to the
return path for the loop battery, LGND and relay drive ground RLYGND.
28 VEE Negative supply rail, -5V.
Functional Description
The MT91600 is the analog SLIC for use in a 4 Wire
switched system. The SLIC performs all of the
normal interface functions between the CODEC or
switching system and the analog telephone line such
as 2W to 4W conversion, constant current feed,
ringing and ring trip detection, current limiting, switch
hook indication and line and network balance
impedance setting using minimal external
components.
Refer to Figure 5 for MT91600 components
designation.
2 Wire to 4 Wire Conversion
The hybrid performs 2 wire to 4 wire conversion by
taking the 4 wire signal from an analog switch or
voice CODEC, a.c. coupled to VR, and converting it
to a 2 wire differential signal at tip and ring. The 2
wire signal applied to tip and ring by the telephone is
converted to a 4 wire signal, a.c. coupled to Vx which
is the output from the SLIC to the analog switch or
voice CODEC.
4W to 2W gain:
Gain 4 - 2 = 20*Log [0.891 * (R14 / R15)]
Impedance Programming
The MT91600 allows the designer to set the device’s
impedance across TIP and RING, (ZTR), and
network balance impedance, (ZNB), separately with
external low cost components.
For a resistive load, the impedance (ZTR) is set by
R11 and R18. For a complex load, the impedance
(ZTR) is set by R11, R18, R19 & C8 (see Figure 5.)
The network balance, (ZNB), is set by R16, R17 & C3
(see Figure 5.)
The network balance impedance should be
calculated once the 2W - 4W gain has been set.
Line Impedance
For optimum performance, the characteristic
impedance of the line, (Zo), and the device’s
impedance across TIP and RING, (ZTR), should
match. Therefore:
Gain Control
It is possible to set the Transmit and Receive gains
by the selection of the appropriate external
components.
The gains can be calculated by the formulae:
2W to 4W gain:
Gain 2 - 4 = 20*Log [ R13 / R12]
Zo = Z
TR
The relationship between Zo and the components
that set ZTR is given by the for mula:
Zo / ( R1+R2) = kZo / R11
where kZo =Z
ZLZ = R18, for a resistive load.
ZLZ = [R18 + (R19 // C8)], for a complex load.
LZ
3
MT91600 Preliminary Information
The value of k can be set by the designer to be any
value between 20 and 250. Three rules to ensure the
correct operation of the circuit:
(A) R18 + R19 > 50kΩ
(B) R1 = R2.
(C) R11 > =50kΩ
It is advisable to place these components as close
as possible to the SLIC.
Network Balance Impedance
The network balance impedance, (ZNB), will set the
transhybrid loss performance for the circuit. The
balance of the circuit is independent of the 4 - 2 Wire
gain but is a function of the 2 - 4 Wire gain.
The method of setting the values for R16 and R17 is
given by the for mula:
R17 = [1.782 * Zo/ ( Zo+ZNB) * ( R13 / R12 )]
R17 + R16 [1 + R13 / R12]
where ZNB is the network balance impedance of the
SLIC and Zo is the line impedance.
The MT91600’s programmable current range is
between 18mA to 32mA.
Line Drivers & Overcurrent Protection
The Line Drivers control the external Battery Feed
circuit which provide power to the line and allows bidirectional audio transmission.
The loop supervision circuitry provides bias to the
line drivers to feed a constant current while the overcurrent 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.
(R16 + R17) >= 50kΩ
It is advisable to place these components as close
as possible to the SLIC.
Loop Supervision & Dial Pulse
Detection
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.
In situations where an accidental short circuit occurs
either across TIP/RING to GND or RING to GND, an
excessive amount of current will flo w 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 datasheet.
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 detecing 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.
4
Preliminary Information MT91600
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.
Application
The following Application section is intended to
demonstrate to the user the methods used in
calculating and selecting the external programming
components in implementing the MT91600 as an
analog line interface in a communication system.
The programming component values calculated
below results in the optimum performance of the
device.
Refer to Figure 5 for MT91600 components
designation.
From Figure 3 with R1 = R2 = 220Ω
For I
V
LC
= 25mA, VLC = 0V, Vbat=-48V
LOOP
R3
43kΩ
R4
C9
100nF
130kΩ
V
BAT
6
V
REF
MT91600
Figure 3 - Resistor Divider
C9 is inserted to ensure pin 6, Vref, remains at a.c.
ground. 100nF is recommended.
I
can also be set by directly driving Vref with a
LOOP
low impedance voltage source. (See Figure 4). It is
recommended that a small resistor be placed in
series with the Vref pin. In this case:
I
= 1.07 * Vs where, Vs < 0
LOOP
(R1 +R2)
Component Selection
Feed Resistors (R1, R2)
The selection of feed resistors, R1 and R2, can
significantly affect the performance of the MT91600.
It is recommended that their values fall in the range
of:
200Ω <= R1 <= 250Ω
where, R1 = R2
The resistors should have a tolerance of 1% (0.15%
matched) and a power rating of 1 Watt.
Loop Current Setting (R3, R4, C9)
By using a resistive divider network, (Figure 3), it is
possible to maintain the required voltage at Vref to
set I
the following relationship:
I
LOOP
(R1 +R2)
where, F = R4 / (R4 + R3)
. The loop current programming is based on
LOOP
= - [ F * VLC + G * V
] * Ko * H
BAT
G = R3 / (R4 +R3)
Ko= 200000 / (200000 + (R4//R3) )
H = 1.07
I
is in Ampere
LOOP
2kΩ
Vs
C9
100nF
6
V
REF
MT91600
Figure 4 - Direct Voltage
Calculating Component Values For AC
Transmission
There are five parameters a designer should know
before starting the component calculations. These
five parameters are:
1) characteristic impedance of the line Z
2) network balance impedance Z
NB
o
3) value of the feed resistors (R1 and R2)
4) 2W to 4W transmit gain
5) 4W to 2W receive gain
The following example will outline a step by step
procedure for calculating component values. Given:
5
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