SEMICMF.019
1
Features
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 driver
Power Denial
Wide Operating Range
Applications
Line interface for:
PABX
Intercoms
Key Telephone Systems
Control Systems
Description
The SLIC provides a complete interface between a
switching system and a 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.
DS5752
Issue 1
May 2002
Ordering Information
MH88612BV-2 20 Pin SIL Package
0
C to 70
C
MH88612B
Subscriber Line Interface Circuit (SLIC)
Data Sheet
Figure 1 - Block Diagram
Current and
Voltage
Sensing
Relay
Drive
Circuit
Constant
Current
Feed
Over Current
Protection
2-Wire to
4-Wire
Conversion
Loop
Supervision
Power
Management
TF
TIP
RING
RF
RV
VRLY
RC
RD
CD
LPGND
VBAT PD VCC VEE AGND
VR
VX
CAP
SHK
VREF
MH88612B
Data Sheet
SEMICMF.019
2
Figure 2 - Pin Connections
Pin Description
Pin #
Name
Description
1
TF
Tip Feed. Connects to external diode for protection
2
LPGND Loop Ground. Is the system ground reference with respect to V
BAT.
Normally connected to
AGND.
3
VR
Voice Receive (input) is the 4 wire analog signal to the device.
4
VREF
Voltage Reference (Input) Constant current feed can be adjusted by the input voltage on
V
REF
. This pin can also be grounded for normal 25mA loop current.
5
VEE
Negative Power Supply Voltage (-5V).
6
AGND
Analog Ground (0V).
7
PD
Power Denial (Input). A logic high will isolate the battery voltage to Tip/Ring.
8
VX
Voice Transmit (Output) is the 4-wire analog signal from the device.
9
TIP
Connects to the "Tip" lead of the telephone line.
10
RING
Connects to the "Ring" lead of the telephone line.
11
RF
Ring Feed (Input) is normally connected to Ring relay for negative battery feed voltage
and ringing voltage input.
12
VCC
Positive Power Supply Voltage (+5V).
13
RC
Relay Control (Input). Active high.
14
RD
Open collector sinks current when RC is at logic high.
15
RV
Ring Feed Voltage connects to pin 11 (RF) through a normally closed relay.
16
VRLY
Relay voltage Supply (+5V).
17
CD
Clamping Diode. Connects to AGND
18
VBAT
Negative Battery Feed Supply Voltage (-48V).
19
CAP
Connects external capacitor to ground for ring trip.
20
SHK
Switch Hook Detect (Output). Active high.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
TF
LPGND
VR
VREF
VEE
AGND
PD
VX
TIP
RING
RF
VCC
RC
RD
RV
VRLY
CD
VBAT
CAP
SHK
Data Sheet
MH88612B
3
SEMICMF.019
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.
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 25mA. 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 defined by the equation:
ILOOP = V
BAT
- V
REF
mA (
+
2 mA)
1.89 1.25
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.
Ringing And Ring Trip Detection
In Figure 3 a ringing signal (eg. 90 Vrms and -48Vdc) is applied to the line by disconnecting pin 15 (RV) from pin
11(RF), 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 1.0
F capacitor connected from pin 19 (CAP) to ground will provide adequate attenuation when ringing is applied.
Once an off-hook condition has been detected a logic low should be applied to pin 13 (RC) which will deactivate
the relay (K1) to disconnect the ringing voltage from pin 11 and reconnect to pin 15. At that time the SLIC will revert
to constant current feed operation.
For applications requiring Dial Pulse Detection the Capacitor connection to ground should be controlled such 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 or system drive output of a codec. Applying AGND to the Ring Trip Filter Control pin as shown in
Figure 4 will switch in the filter, whilst removing AGND, (with the switch in a tri-state condition), will switch out the
filter.
For applications using DTMF signalling, the capacitor can be permanently connected to ground.
MH88612B
Data Sheet
SEMICMF.019
4
Current Limit
The Tip or Ring of the telephone line may accidentally be shorted 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.
Power Denial
The power denial function is a feature of the MH88612B which allows for powering down of the subscriber loop. A
logic high voltage applied to the power denial input effectively removes the battery voltage from the loop driver
circuitry. The resulting I
LOOP
is negligible and power consumption is minimized. The power denial function is useful
for disabling a loop which may have a ground fault.
Data Sheet
MH88612B
5
SEMICMF.019
Figure 3 - Application Circuit
VR
VX
SHK
PD
CAP
AGND
VEE
VCC
VBAT
CD
RD
RC
VRLY
RING
TIP
TF
LPGND
RV
RF
VBAT
3
8
20
7
19 C1
6
5
12
18
-5V
+5V
-48V
17
14
16
13
10
9
1
2
15
11
D3
D1
D2
D4
TIP
RING
+5V
K1
R1
+
-
MH88612B
K1
R2
~
90 VRMS
-
+
48V
Protection
Circuit
C2
C3
Q1
R3
RC
(Pin 13)
R4
Components List
R1
200
10%, 1/4W
R2
470k
5%, 1/4W
R3, R4
10k
5%, 1/4W
C1
1.0
f,
10%, 25V electrolytic capacitor
C2, C3
0.1
f
20%, 50V ceramic de-coupling capacitors
D1, D2, D3, D4
Diode rectifier 200V, 1A, IN4003
Q1
General Purpose switching transistor
K1
Relay E/M, 5V or 6V 1Form C
Protection Circuit - depending on the application the Protection Circuit may need to provide
over-voltage and/or over-current protection. In certain circumstances no protection will be required.
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