PBL 3766
The internally set loop current detector
threshold, ILThOn, for the off-hook to on-
hook transition is 7.3 mA.
Ring Trip Detector
output to toggle between the on-hook and
off-hook states at the ringing frequency.
However, with the telephone off-hook, the
DET output will be at logic low level for
more than half the time. Therefore, by
sampling the DET output, a software
routine can discriminate between on-hook
and off-hook through examination of the
duty cycle. Full removal of the ringing
frequency from the DT input, while
maintaining ringtrip within required time
limits (approximately < 100 ms), usually
mandates a second order filter rather than
the first order shown in figure 15. The
software approach minimizes the number
of line card components.
In the balanced ringing system shown
in figure 16, RRT1 and RRT2 are the ringing
feed and loop current sensing resistors.
With the telephone on-hook, no dc loop
current flows to cause a dc voltage drop
across resistor RRT1. Voltage divider R1, R2
and R3 biases the ringtrip comparator
input DT to be more positive than VEE
during on-hook. With the telephone off-
hook during ringing dc loop current will
flow, causing a voltage drop across
resistor RRT1, which will make comparator
input DT more negative than VEE. This will
set the DET output to logic low level,
indicating ringtrip condition. Capacitors C1
and C2 filter the ringing voltage at the
comparator input. With component values
according to figure 16, 20 Hz ringing will
be attenuated by 20 dB and 30 Hz ringing
will be attenuated by 23 dB before reach-
ing the DT input.
Ring trip detection is accomplished by
monitoring the two-wire line for presence
of dc current while ringing is applied.
When the subscriber goes off-hook during
ringing, dc loop current starts to flow. The
SLIC ring trip comparator detects this
current flow via an interface network. The
DT comparator input is connected to pin
23/20. The other comparator input is
internally connected to VEE. The result of
the comparison is presented at the DET
output with logic low level indicating off-
hook. The ring trip comparator is
automatically connected to the DET
output, when the SLIC control inputs are
set to the ringing state (C2, C1 = 0, 1).
When off-hook during ringing is detected,
the line card or system controller will
proceed to disconnect the ringing source
(software ringtrip) by re-setting the control
input logic states. Alternatively, the DET
output may be monitored by circuits on
the line card, which perform the ringtrip
function (hardware ringtrip).
The ringing source may be balanced or
unbalanced, superimposed on the VBat
supply voltage. A ring relay, energized by
the SLIC ring relay driver, connects the
ringing source to tip and ring. For unbal-
anced ringing systems the loop current
sensing resistor, RRT, is placed in series
with the return lead to ground.
Figures 15 and 16 show examples of
unbalanced and balanced ringing
systems. For either ringing system the
ringtrip detection function is based on a
An external resistor, RD, may be
connected from terminal RD to VEE to
increase the loop current detector
thresholds. When the desired on-hook to
off-hook loop current threshold, ILThOff, is
known, the RD value is calculated from
1
RD =
ILThOff /500 - 1/62500
where RD is in ohms for ILThOff in amperes
The off-hook to on-hook loop current
detector threshold, ILThOn, for the selected
RD value is calculated from
1
1
ILThOn = KLThOn
·
+
RD 62500
where
ILThOn is in amperes for RD in ohms.
ILThOn > 7.3 mA, KLThOn = 455 V
The on-hook to off-hook loop current
detector threshold, ILThOff, for a specific RD
value is calculated from
1
1
ILThOff = KLThOff
·
+
RD 62500
where
ILThOff is in amperes for RD in ohms.
ILThOff > 8.0 mA, KLThOff = 500 V
With a lower voltage battery it may be
desirable to decrease the loop current
detector thresholds. For more information
on this issue, please contact the factory.
During dial pulsing the loop current
detector is aided by a speed-up circuit,
acting on the RDC output at loop closures.
The speed-up circuit will charge the CDC
capacitor at a more rapid rate than that set
by the (CDC· RDC· 41700)/(RDC+ 41700)
time constant, resulting in the loop current
reaching the detector threshold value
faster and therefore minimizing dial pulse
distortion.
polarity change at the inputs of the ringtrip Relay Driver
comparator.
The PBL 3766 SLIC incorporates a ring
In the unbalanced case the dc voltage
drop across resistor RRT is zero, as long
as the telephone remains on-hook. With
the telephone off-hook during ringing, dc
loop current will flow, causing a voltage
drop across RRT. The RRT voltage is
relay driver designed as open emitter with
grounded collector (npn) having a current
sourcing capability of 50 mA. The relay
coil must be connected to a negative
voltage ≤ |VBat|. An external inductive kick-
back clamp diode must be employed to
protect the drive transistor.
applied to the comparator input DT via
resistor R1. R2 shifts the voltage level to
Loop Current Detector - Filter Capacitor be compatible with the inverting input VEE Control Inputs
reference voltage. CRT removes part of
To increase the loop current detector
Overview
the ac component of the ringing signal.
noise immunity, a filter capacitor may be
The inverting comparator input is
biased at VEE, which is more negative
than DT when the telephone is on-hook
and is more positive than DT when the
telephone goes off-hook during ringing.
Complete removal of the ringing signal
ac component at the DT input is not
necessary. Some residual ac component
at the DT input may, under certain
operating conditions, cause the DET
The PBL 3766 SLIC has two TTL
added from terminal RD to ground. A
suggested value for CD is
compatible control inputs, C1 and C2. A
decoder in the SLIC interprets the control
input conditions and sets up the com-
manded operating state.
RD + 62500
CD =
2π · (RD · 62500) · f3dB
where
Open Circuit State (C2, C1 = 0, 0)
CD is in farads for RD in ohms f3dB = 500 Hz
In the Open Circuit State the TIPX and
RINGX line drive amplifiers as well as
other circuit blocks are powered down.
Note that CD may not be required if the
DET output is software filtered.
4-14
ERICSSON [ ERICSSON ]
