PBL 3764A/4, /6
results at the DET (pin 11) output when
the loop current exceeds the on-hook to
monitored by the ring trip comparator
input DT via the network R3, R4 and CRT.
Input DR is set to a reference voltage by
examined by a software routine to
determine the duty cycle. When the DET
output is at logic level low for more than
off-hook detect threshold, ILThOff
:
The programming resistor RD value can
be calculated for a desired ILThOff from
RD = 465/ILThOff. RD is in kΩ for ILThOff in
mA.The off-hook to on-hook threshold,
ILThOn, for a known RD is ILThOn = 405/RD.
A logic high results at the DET output
resistors R1 and R2. With the line on-hook half the time, off-hook condition is
(no dc current) DT is more positive than
DR and the DET output will report logic
level high, i.e. the detector is not tripped.
When the line goes off-hook, while
ringing, a dc current will flow through the
loop including sense resistor RRT and will
cause input DT to become more negative
indicated.
Relay Driver
The PBL 3764A/4 SLIC incorporates a
ring relay driver designed as open
collector (npn) with a current sinking
capability of 50 mA. The drive transistor
emitter is connected to BGND. An
external inductive kick-back clamp diode
must be employed to protect the drive
transistor.
when the loop current is less than ILThOn
.
The CD filter capacitor is calculated
according to CD = T/RD with time constant than input DR. This changes output DET
T = 0.5 ms. Note that CD may not be
required if DET is software filtered.
to logic level low, i.e. tripped detector
condition. The system controller (or line
card processor) responds by de-
Ground Key Detector
energizing the ring relay, i.e. ring trip
Complete filtering of the 20 Hz ac
component at terminal DT is not
Control Inputs
The PBL 3764A/4 SLIC has two TTL
compatible control inputs, C1 and C2.
A decoder in the SLIC interprets the
The ground key detector circuit examines
the difference in TIPX and RINGX
currents. Should the current difference
exceed the threshold value, ∆ILOn, the
detector is triggered. As the current
difference decreases the detector is reset
necessary. A toggling DET output can be
16
at current threshold ∆ILOff. ∆ILOn > ∆ILOff
i.e. the detector has hysteresis. The
,
C
1
Ground
Key Det.
Input
Decoder
15
12
C
E
2
1
I
LTIPX
triggered detector results in a logic low at
the DET (pin 11) output, assuming the
ground key detector has been selected
via the four-bit control input (C1, C2, E0,
E1). For ∆ILOn and ∆ILOff numerical values
please refer to table “Electrical character-
istics”.
TIPX
V
2-Wire
Interface
CC
5
6
RINGX
I
- I
LRINGX
I
LTIPX
LRINGX
2
300
14
13
DET
+
MUX
-
E
0
1.25V
PBL 3764A/4
2
20
V
Ring Trip Detector
R
D
EE
Ring trip detection is accomplished by
connecting an external network to a
comparator in the SLIC with inputs DT
(pin 23) and DR (pin 25). The ringing
source can be balanced or unbalanced
superimposed on VBat. The unbalanced
ringing source may be applied to either
the ring lead or the tip lead with return via
the other wire. A ring relay driven by the
SLIC ring relay driver connects the ringing
source to tip and ring.
R
C
D
D
-
5V
Figure 21. Loop current and ground key detectors.
C
R
R
1
RT
RT
The ring trip function is based on a
polarity change at the comparator input
when the line goes off-hook. In the on-
hook state no dc current flows through the
loop and the voltage at comparator input
DT is more positive than the voltage at
input DR. When the line goes off-hook,
while the ring relay is energized, dc
current flows and the comparator input
voltage reverses polarity.
R
3
DT
DR
3
4
-
+
R
R
4
2
TIP
E
PBL 3764A/4
RG
K
R
Figure 22 is an example of a ring trip
detection network. This network is
applicable, when the ring voltage
V
Bat
RING
superimposed on VBat is injected on the
ring lead of the two-wire port. The dc
voltage across sense resistor RRT is
Figure 22. Ring trip network.
16
ERICSSON [ ERICSSON ]
