®
MH88612C
Subscriber Line Interface Circuit
Preliminary Information
Features
•
•
•
•
•
•
•
•
•
•
Input impedance 200Ω
+ 560Ω // 0.1µF
Transformerless 2-wire to 4-wire conversion
Battery and ringing feed to line
Off-hook and dial pulse detection
Ring ground over-current protection
Adjustable constant current feed
Relay drive output
Power VBat Denial
Wide Operating Range
Programmable Receive Gain
MH88612C
ISSUE 4
April 1995
Ordering Information
20 Pin SIL Package
0°C to 70°C
Description
The SLIC provides a complete interface between a
switching system and subscriber loop. Functions
provided include battery feed and ringing feed to the
subscriber line, 2-Wire to 4-Wire hybrid interfacing,
constant current feed and dial pulse detection. The
device is fabricated using thick film hybrid
technology in a 20-pin single in-line package.
Applications
Line interface for:
•
•
•
•
PABX
Intercoms
Key Telephone Systems
Control Systems
VBat
VDD VEE GNDA
GS
TF
TIP
RING
RF
RV
VRLY
RC
RD
Current & Voltage
Sensing
2-Wire to
4-Wire
Conversion
VR
VX
Over Current
Protection
Relay
Drive
Circuit
Loop
Supervision
VRef
CAP
Constant
Current
Feed
SHK
FIgure 1 - Functional Block Diagram
2-113
MH88612C
IC
TF
IC
VR
VRef
VEE
GNDA
GS
VX
TIP
RING
RF
VDD
RC
RD
RV
VRLY
IC
VBat
CAP
SH K
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Preliminary Information
Figure 2 - Pin Connections
Pin Description
Pin #
0
1
2
3
4
Name
IC
TF
IC
VR
VRef
Description
Internal Connection.
No pin fitted in this position.
Tip Feed.
Connects to external diode to LPGND for protection.
Internal Connection.
No pin fitted in this position.
Voice Receive
(input) is the 4 wire analog signal to the SLIC.
Voltage Reference
to set the constant current feed to the subscriber line. Constant current
feed can be adjusted by the input voltage on the VRef. This pin can also be grounded for
normal 21mA loop current.
Negative Power Supply voltage.
(-5V).
Analog Ground.
(0V)
Gain setting (Input).
A logic low at this input adds an additional -0.5dB gain in the receive
direction. This gain is in addition to the gain set by the Codec. A high adds 0dB gain.
Voice Transmit
(Output) is the 4-wire analog signal from the SLIC.
Connects to the “Tip” lead of the telephone line.
Connects to the “Ring” lead of the telephone line.
Ring Feed (Input).
is connected via the ring relay to either negative battery feed voltage,
(RV), or ringing voltage input.
Positive Power Supply Voltage.
(+5V)
Relay Control (Input).
Active high will apply ringing to the line.
Output Drive Sources current when RC is at a logic high.
Ring Feed Voltage
connects to pin 11 (RF) through a normally closed relay contact.
Relay voltage Supply.
(+5V)
Internal Connection.
No pin fitted in this position.
Negative Battery Feed Supply Voltage.
(-48V)
Connects external capacitor to ground for ring trip.
Switch Hook Detect (Output).
Active high.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
VEE
GNDA
GS
VX
TIP
RING
RF
VDD
RC
RD
RV
VRLY
IC
VBat
CAP
SHK
2-114
Preliminary Information
Functional Description
The SLIC performs a transformerless 2-wire to
4-wire conversion of the analog signal. The 2-wire
circuit is the balanced line going to the subscriber
loop, while the 4-wire circuit is the audio signal going
to and from devices such as the voice codec or
switching circuit. The SLIC also provides a switch
hook (SHK) status output which goes high when the
telephone is set off-hook.
MH88612C
For applications requiring dial pulse detection the
capacitor connection to ground should be controlled
so that the capacitor is only connected during
ringing.
During off-hook conditions (closed loop), the
capacitor should be switched out. This can be
performed using a transistor, relay to system drive
output of a codec. Applying GNDA to Ring Trip Filter
Control pin will switch in the filter, whilst removing
GNDA, (with the switch in a tristate condition), will
switch out the filter.
For applications using DTMF signalling, the
capacitor can be permanently connected to ground.
Once an off-hook condition has been detected a
logic low on the Relay Control (RC) input deactivates
the relay (K1) to disconnect the ringing voltage from
pin 11 (RF)and reconnect to pin 15 (RV). At that time
the SLIC will revert to constant current feed
operation.
Constant Current Feed
The SLIC employs a complex feedback circuit to
supply a constant feed current to the line. This is
done by sensing the sum of the voltages across the
internal feed resistors and comparing it to an input
reference voltage (V
Ref
) that determines the constant
feed current. If V
Ref
(pin 4) is externally grounded,
the constant current is set at 21mA. For a constant
current setting between 18mA and 30mA, V
Ref
can
be set between -8.75 Vdc and -6.35 Vdc. The
relationship is detailed in the formula below:
I
Loop
= [-2.6(V
Ref
) + 21.5] mA +2mA
Current Limit
The Tip or Ring may accidentally short to ground. In
such a case, current will only flow through the feed
resistor. This high current will be sensed and
reduced by the current limit circuit to a lower value to
protect the internal circuitry.
Switch Hook Detection
When the DC current exceeds an internal threshold
level, the switch hook (SHK) will go high. If the loop
resistance is so high that V
Bat
can no longer supply
the required amount of loop current as determined
by constant current supply circuit, the output of the
switch hook (SHK) will go low to indicate that the
loop resistance is too high and the line is on hook.
Receive Gain
The MH88612C Receive Gain (VR to 2-wire) can be
adjustable by -0.5dB if GS is taken logic low or by
0dB if GS is taken logic high. When the MH88612C
issued with an off hybrid MT896x codec the Receive
Gain can be set to either -3.5dB or -7dB by writing to
control Register A of Codec and programming GS
appropriately.
Ringing And Ring Trip Detection
In Figure 3 a ringing signal (e.g. 90 Vrms
superimposed on -48 VDC) is applied to the line by
disconnecting pin 15 (RV) from pin 11 (FR), and
connecting the ringing voltage at pin 11 (RF) by use
of the relay K1.
The SLIC can detect an off-hook condition during
ringing but there is a large AC component which
must be filtered out to give a true off-hook condition
at SHK.
A 1µF capacitor connected from pin 19 (CAP) to
ground will provide adequate attenuation when
ringing is applied.
2-115
MH88612C
Applications
Component List
Preliminary Information
= 1/4 W, + 10% 200Ω
= 1/4W, ±5%, 470KΩ
= Cap. Elec., 1.0µF, + 10%, 25V
= Diode Rect., 200V, 1A, IN4003
= Relay E/M, 5 or 6V, 1 form C
Decoupling capacitors (Cap. Cer 0.1µF, 50% + 20%) should be added to V
DD
, V
EE
, V
Bat
, V
Ref
and V
RLY
.
Options RV1, RV2= 160 Vrms, 250V 10J
R1
R2,
C1
D1,D2
K1
90 VRMS
SUPERIMPOSED
ON -48VDC
VBat
R1
K1
11
D4
D1
D2
MH88612C
RF
15
RV
3
VR
1
D3
TF
VX
8
VBat
TIP
9
RV1
4
VRef
20
RV2
SHK
RING
RC
13
+5V
16
K1
10
7
GS
VRLY
RD
VBat VDD
18
12
VEE
5
CAP
GNDA
6
19
-
C1
Ring Trip
Filter
Control
14
+
R2
-48V
+5V
-5V
Figure 3 - Application Circuit
2-116
Preliminary Information
Absolute Maximum Ratings*
Parameter
1
DC Supply Voltage
Symbol
V
DD
V
EE
V
Bat
T
STG
P
D
Min
-0.3
+0.3
0.3
-40
Max
+15
-15
-60
100
MH88612C
Units
V
V
V
°C
2
3
Storage Temperature
Package Power Dissipation
2
w
* Exceeding these values may cause permanent damage. Functional operation under these conditions is not implied.
Recommended Operating Conditions
Parameter
1
Operating Supply Voltage
Sym
V
DD
V
EE
V
Ref
V
Bat
V
RLY
2
Operating Temperature
T
O
0
* Typical figures are at 25°C and are for design aid only.
Min
4.75
-4.75
-27
Typ*
5.0
-5.0
0
-48
5
Max
5.25
-5.25
-60
70
Units
V
V
V
V
V
°C
Comments
Adjustable
DC Electrical Characteristics
†
Characteristics
1
Supply Current
-
Voltages are with respect to GNDA unless otherwise stated.
Sym
V
DD
V
EE
VBat
I
DD
I
EE
I
BAT
P
C
I
Loop
I
Loop
R
Loop
Min
0.8
0.5
0.0
Typ*
5
3
2
210
1410
Max
12
10
2.5
Units
mA
mA
mA
mW
mW
Test Conditions
Idle State
Standby
Active
V
Ref
= GNDA,
V
Bat
= -48V
Adjustable with Ext.
V
Ref
input, V
Bat
= -48V
V
Bat
= -48V @ I
Loop
=
18mA, R
Loop
includes
telephone set
2
3
4
5
Power Consumption
Constant Current Line Feed
Adjustable Loop Current Range
Operating Loop Resistance
18
18
21
24
30
1850
mA
mA
Ω
6
7
Ring Ground Over-Current
protection
Off-Hook Detect
Output Low Current (On-hook)
Output High Current (Off-hook)
Off- Hook Detect
Output Low Current (on-hook)
Input High Current. (activation)
RC Control Input
Input Low Volt. (no activation)
Input High Volt. (activation)
RC control Input
Input Low Current (no
activation)
Input high current (activation)
V
OL
V
OH
I
OL
I
OH
V
IL
V
IH
I
IL
I
IH
0.0
2.7
40
mA
Active high logic
0.4
5.25
8
-400
0.7
V
V
mA
µA
Active high logic
V
V
µA
µA
8
9
2.0
-50
50
10
† DC Electrical Characteristics are over recommended operating supply voltages.
* Typical figures are at 25°C with nominal +/-5V supplies and are for design aid only.
2-117
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