PBL 386 11/2
Ground Key Detector
lead of the two-wire port. The dc voltage
across sense resistor RRT is monitored by
the ring trip comparator input DT and DR
via the filter network R1, R2, R3, R4, C1 and
C2. DT is more positive than DR, with the
line on-hook (no dc current). The DET
output will report logic level high, i.e. the
detector is not tripped. When the line goes
off-hook,whileringing,adccurrentwillflow
through the loop including sense resistor
Relay driver
ThePBL38611/2 SLICincorporatesaring
relay driver designed as open collector
(npn) with a current sinking capability of 50
mA.The emitter of the drive transistor is
connected to BGND. The relay driver has
aninternalzenerdiodeclamptoprotectthe
SLICfrominductivekick-backvoltages.No
external clamp is needed.
The ground key detector indicates when
the ground key is pressed (active) by put-
ting the output pin DET to a logic high level
when selected. The ground key detector
circuitsensesthedifferencebetweenTIPX
and RINGX currents. The detector is trig-
gered when the difference exceeds the
current threshold.
R
RT and will cause the input DT to become
Control Inputs
Ring Trip Detector
more negative than input DR. This chang-
es the output on the DET pin to logic level
low, i.e. tripped detector condition. The
system controller (or line card processor)
responds by de-energizing the ring relay
via the SLIC, i.e. ring trip.
Completefilteringofthe20Hzaccompo-
nent at terminals DT and DR is not neces-
sary. A toggling DET output can be exam-
ined by a software routine to determine the
duty cycle. Off-hook condition is indicated
whentheDEToutputisatlogiclevellowfor
more than half the time.
The PBL 386 11/2 SLIC has three digital
control inputs, C1, C2 and C3.
A decoder in the SLIC interprets the con-
trol input condition and sets up the com-
manded operating state.
Ring trip detection is accomplished by con-
necting an external network to a compara-
tor in the SLIC with inputs DT and DR. The
ringing source can be balanced or unbal-
anced e g superimposed on the battery
voltage or ground. 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.
The ring trip function is based on a polar-
ity 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
positivethanthevoltageatinputDR. When
the line goes off-hook, while the ring relay
is energized, dc current flows and the com-
parator input voltage reverses polarity.
Figure 13 gives an example of a ring trip
detection network. This network is applica-
ble, when the ring voltage superimposed
onthebatteryvoltageisinjectedonthering
C1 to C3 are internal pull-up inputs.
Open Circuit State
In the Open Circuit State the TIPX and
RINGXlinedriveamplifiersaswellasother
circuitblocksarepowereddown.Thiscaus-
es the SLIC to present a high impedance to
the line. Power dissipation is at a minimum
and no detectors are active.
Detector Output (DET)
The PBL 386 11/2 SLIC incorporates a
detector output driver designed as open
collector (npn) with a current sinking capa-
bility of min 3 mA, and a 10 kΩ pull-up
resistor. The emitter of the drive transistor
is connected to AGND. A LED can be
connected in series with a resistor (≈1 kΩ)
at the DET output to visualize, for example
loop status.
Ringing State
In the ringing state the SLIC will behave
as in the active state with the exception
that the ring relay driver and the ring trip
detector are activated. The ring trip detec-
tor will indicate off hook with a logic low
level at the detector output.
15
ERICSSON [ ERICSSON ]
