2-11
Features
FAX and Modem interface
Suitable for V.29 FAX & V.22 bis Data
Allows caller identification
Variants match German, UK, 600
& 900
network impedances
Isolation circuitry conforms to international PTT
requirements
Transformerless 2-4 Wire conversion
Loop start operation
Pulse and DTMF operation
Accommodates external monitor phone
Line state detection outputs:
-loop current/ringing outputs
-monitor phone switch hook
Single +5V operation
Applications
Interface to Central Office for:
DAA
Modem
FAX
Answering Machine
Terminal Equipment
Description
The Mitel MH88400 Line Interface Circuit provides a
complete audio and signalling link between audio
equipment and central office. The functions provided
by the MH88400 include 2-4 Wire conversion, loop
seizure, external monitor phone switch hook status
and ringing voltage and loop current detection. The
device is fabricated as a thick film hybrid which
incorporates various technologies for optimum circuit
design, high voltage isolation and very high
reliability.
Variants are provided to meet German (-1) and UK
(-3) and general 600
(-2) line impedance (see
section on Line Impedances).
Ordering Information
MH88400-1
14 Pin DIL Package
MH88400-2
14 Pin DIL Package
MH88400-3
14 Pin DIL Package
0
C to 70
C
Power
Supply
Opto-
Isolation
Logic Input
Buffer
Opto-
Isolation
Opto-
Isolation
Audio
Buffer
Audio
Buffer
Ring &
Buffer
Active
Termination
Audio Input
Buffer
Input
Buffer
Isolation Barrier
VDD
AGND
LC
VR
VX
RVLC
TIP
RING
TXIN
TF
RLS
USER CONNECTIONS
NETWORK CONNECTIONS
Opto-
Isolation
PHS
SHK
Switch Hook
Buffer
Opto-
Isolation
Transformer
Isolation
Loop
Figure 1 - Functional Block Diagram
MH88400
Line Interface Circuit
Preliminary Information
ISSUE 1
April 1995
2-12
MH88400
Preliminary Information
Figure 2 - Pin Connections
Pin Description
Pin #
Name
Description
1
VDD
Positive Power Supply Voltage. +5V.
2
AGND
Analog Ground. 4-Wire Ground. Normally connected to System Ground,
3
LC
Loop Control (Input). A logic low activates internal circuitry which provides a dc
termination across Tip and Ring. Used for seizing the line and dial pulsing
4
RVLC
Ring Voltage and Current Detect (Output). A logic low indicates that loop current is
detected. The loop current can be due to the external monitor phone or the MH88400 in
the off- hook mode. The RVLC outputs pulses when the external monitor phone is dial
pulsing or when the MH88400 is dial pulses via the LC input. In addition, when the
MH88400 is in the on-hook mode, a pulsing output indicates that ringing voltage is
across the Tip and Ring leads; the pulsing outputs frequency is twice the ringing
frequency.
5
SHK
Switch Hook Detect (Output). This is an optional output which can be used with the
PHS input and an external phone. When loop current flows from PHS to TIP, SHK goes
to logic low.
6
VX
Transmit (Output). 4-Wire ground(AGND) referenced audio output, biased at 2.5V.
Outputs for both off-hook and on-hook.
7
VR
Receive (Input). 4-Wire ground (AGND) referenced audio input, biased at 2.5V. LC
must be activated low and loop current must be flowing.
8
NC
No Connection. This pin is not connected internally
9
RING
Ring Lead. Connects to the "Ring" lead of the central office through a relay contact. The
central office "Tip" and "Ring" leads may be interchanged
10
TXIN
Transmit (Input). Connects to the "Ring" lead of the central office through a coupling
capacitor.
11
TF
Tip Feed. Connects to the "Tip" lead of the central office through an internal resistor and
an optional external resistor.
12
PHS
Monitor Phone Sense (Input). This is an optional input which can be used with the
SHK output and an external phone. When loop current flows from PHS to TIP, SHK goes
to logic low.
13
RLS
Ring Loop Sense (Input). Sense node for ringing voltage detector and the loop current
detector.
14
TIP
Tip Lead. Connects to the "Tip" lead of the central office through an optional relay
contact. The central office "Tip" and "Ring" leads may be interchanged.
RLS
PHS
TF
TXIN
LC
NC
TIP
AGND
RING
RVLC
SHK
VX
VR
VDD
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Preliminary Information
MH88400
2-13
Functional Description
The MH88400 Line Interface Circuit is a COIC
(Central Office Interface Circuit) used to interface
FAX's Modems or user defined equipment to
Central Office 2-Wire Analog Trunks.
Opto-Isolation
The isolation barrier is designed to meet regulatory
requirements for a reinforced barrier of 3kVac. It
provides full isolation of mains voltages up to 250V
RMS and all telecom voltages. In order that this
barrier is not bypassed a creepage/clearance
distance of 6.4mm minimum must be maintained
between wiring, pcb tracking, etc., connected to
external circuitry on either side of the barrier. To
make this requirement simpler, pins on the
MH88400 connected to opposite sides of the
barrier are on opposite sides of the package.
External Protection Circuit
To meet regulatory high voltage requirement, an
external protection circuit is required. The
protection circuit shown in Figure 3 (Clamp Diode
D1) is recommended.
DC Loop Termination
The DC loop termination circuitry provides the loop
with an active DC load termination when a logic low
is applied to the LC (Loop Control) input. The
termination is simpler to a resistance of
approximately 300
(loop current dependant).
Internal optically isolated circuitry is used to switch
the termination in and out the loop. This is used for
both seizing the line as well as generating dial
pulses.
Supervision Features
The supervision circuitry is capable of detecting
ringing voltage and loop current as well as the
status of an optional external monitor phone. The
RVLC (Ring Voltage Loop Current Detect) output
provides a logic low when loop current due to the
external monitor phone or due to the MH88400
being in the off-hook mode is detected.
The RVLC outputs pulses when the external
monitor phone is dial pulsing or when the MH88400
is dial pulsing via the LC mode.
In addition, when the MH88400 is in the on-hook
mode, a pulsing output indicates that ringing voltage
is across the tip and ring leads; the pulsing output
frequency is twice the ringing frequency. See
Figure 3.
Ringing frequency may require external validation.
An RC monostable is usually satisfactory for this
purpose or this may also be achieved using software
applications.
2-4 Wire Conversion
The 2-4 Wire conversion circuit converts the
balanced full duplex signal at Tip and Ring of the
central office line into a transmit ground referenced
signal at VX (Transmit) of the MH88400. It also
converts the receive ground referenced signal at VR
(Receive) of the MH88400 into a balanced transmit
signal at Tip and Ring of the central office line.
In full duplex transmission, the Tip-Ring signal
consists of an audio signal from the central office as
well as an audio signal due to the VR input.
Consequently, both of these signals will appear at the
VX output. The degree to which the 2-4 wire
conversion circuit minimizes the contribution of the
VR signal at the VX output is specified as transhybrid
loss (THL).
A simple THL cancellation circuit as shown in figure 7
can be used for certain applications to give the
required VX/VR signal separation. The MH88400 is
then suitable to drive a COMBO 2 CODEC or a VLSI
MODEM device.
Line Impedance
The MH88400 is suitable to drive a COMBO 2
CODEC or a VLSI modem device. The MH88400
provides a fixed Tip-Ring impedance which conforms
to the following PTT requirements.
MH88400-1 Zin = 200 + 820 // 115nF
Germany FTZ
MH88400-2 600
MH88400-3 Zin = 370 + 620 // 310nF
UK BSI
2-14
MH88400
Preliminary Information
.
* Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied.
Typical figures are at 25C with nominal +5V supplies and are for design aid only.
Loop Electrical Characteristics*
* Loop Electrical Characteristics are over recommended operating conditions unless otherwise stated.
Typical figures are at 25C and are for design aid only.
Note: All of the above characteristics use a test circuit as per Figure 3.
Absolute Maximum Ratings* -
All voltages are with respect to AGND unless otherwise specified.
Parameter
Symbol
Min
Max
Units
1
DC Supply Voltage
V
DD
-0.3
6
V
2
Storage Temperature
T
S
-55
+80
C
3
DC Loop Voltage
V
BAT
-100
+100
V
4
Ringing Voltage
V
R
-
120
V
RMS
5
Loop Current
I
Loop
-
90
mA
Recommended Operating Conditions
Parameters
Sym
Min
Typ
Max
Units
Test Conditions
1
DC Supply Voltages
V
DD
4.5
5.0
5.5
V
2
Operating Temperature
T
OP
0
70
C
3
Ringing Voltage
V
R
75
V
RMS
V
BAT
=-48V
Characteristics
Sym
Min
Typ
Max
Units
Test Conditions
1
Ringing Voltage No Detect
Detect
VR
27
20
V
RMS
V
RMS
2
Ringing Frequency
15
68
Hz
3
On-Hook 2-wire Impedance
40k
1kHz
4
Operating Loop Current
10
80
mA
5
Operating Loop Resistance
4000
V
BAT
=-48V, I
Loop
=10mA
6
Off-Hook DC -1 & -2 Variants
Resistance
-3 Variant
310
160
350
300
250
400
480
480
300
450
I
Loop
=20mA
I
Loop
=40mA
I
Loop
=40mA
I
Loop
=20mA
7
Leakage Current (2-Wire to AGND)
10
A
100V
DC
8
Leakage Current on Hook
(Tip to Ring)
9
10
A
V
BAT
=50V
9
DC Resistance during dialling
-1 variant
200
220
I
Loop
=20-40mA
10
Dial Pulse Distortion ON
Off
0
+4
0
+8
+2
ms
ms
2-15
Preliminary Information
MH88400
DC Electrical Characteristics are over Recommended Operating Conditions unless otherwise stated.
Typical figures are at 25
C with nominal+ 5V supplies and are for design aid only.
Note: See figure 3a and 3b.
AC Electrical Characteristics are over Recommended Operating Conditions unless otherwise stated.
Typical figures are at 25
C and are for design aid only.
Note 1: All of the above characteristics use a test circuit as per Figure 3.
Note 2: All of the above test conditions use a test source impedance which matches the device's impedance.
Note 3: dBm is referenced to 600
unless otherwise stated.
Note 4: THD is measured with a "Weight" filter
.
DC Electrical Characteristics
Characteristics
Sym
Min
Typ
Max
Units
Test Conditions
1
RVLC
SHK
Supply Current
I
DD
15
mA
V
DD
= 5.0V, I
Loop
=
40mA
2
Power Consumption
PC
75
mW
V
DD
= -5.0V,I
Loop
=
40mA
3
Low Level Output Voltage
High level Output Voltage
V
OL
V
OH
3.8
0.4
V
V
I
OL
= 1.0mA
I
OH
= 1.0mA
4
Low Level Input Voltage
High level Input Voltage
High Level Input Current
Low Level Input Current
V
IL
V
IH
I
IH
I
IL
3.5
0.9
0.6
1
V
V
m
A
V
IH
= 5.0V
V
IL
= 0.0V
5
AC Electrical Characteristics
- MH88400 All Variants
Characteristics
Sym
Min
Typ
Max
Units
Test Conditions
1
Input Impedance VR
47
k
2
Output Impedance at VX
5
3
Transmit Gain (2-Wire to VX)
-0.4
dB
Input 0.5V at 1kHz off-
hook
4
Frequency Response Gain
(relative to Gain @ 1kHz)
-2.4
-3.0
dB
dB
300 Hz
3400 Hz
5
Receive Gain (VR to 2-wire)
3.5
dB
Input 0.5V at 1kHz
6
Frequency Response Gain
(relative to Gain @ 1kHz)
0
0
dB
dB
300 Hz
3400 Hz
7
Signal Output Overload Level
at 2-Wire
at Vx
THD <5% @ 1kHz
I
Loop
=10-40mA
-3.0
0.0
+2.0
dBm
dBm
dBm
V
DD
=4.5V
V
DD
=5.0V
V
DD
=5.5V
-3.0
0.0
+2.0
dBm
dBm
dBm
V
DD
=4.5V
V
DD
=5.0V
V
DD
=5.5V
8
Total Harmonic Distortion
at 2-wire
at VX
THD
0.9
1.2
%
%
Input 0.5V at 1kHz
DC loop = 1000
V
DD
=5.0V
9
Power Supply Reject Ratio
at 2-wire
at VX
PSRR
35
10
dB
dB
Ripple 0.1V,1kHz on
V
DD
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