HC5517_00 Datasheet, PDF (13/19 Pages) Intersil Corporation – 3 REN Ringing SLIC For ISDN Modem/TA and WLL

HC5517
TABLE 3. HC5517 APPLICATION CIRCUIT OPERATING MODES SUMMARY
BATTERY
SWITCH
F1
RC
MODE
DETECTORS VALID
SHD
RTD
APPLICATION CIRCUIT VALID
MTU
CENTERING
-24V
0
0
Power Denial
-24V
0
1
Invalid
-24V
1
0
Off-Hook Active
√
-24V
1
1
Invalid
-80V
0
0
Power Denial
√
-80V
0
1
Invalid
-80V
1
0
On-Hook Standby
√
√
-80V
1
1
Ringing
(Note)
√
√
NOTE: During Ringing, the SHD output will be active for both on-hook and off-hook conditions. The AC current, for the on-hook condition, exceeds
the SHD threshold of 12mA. Valid off-hook detection during ringing is provided by the RTD output only.
Operation of the Battery Switch
The battery switch is used to select between the off-hook
battery of -24V and the ringing/standby battery of -80V.
When T₁ is off (battery switch is logic low) the MOSFET T₃
is off and the -24V battery is supplied to the SLIC through
D₁₀. When T₁ is on (battery switch is logic high) current
flows through the collector of T₁ turning on the zener D₉.
When D₉ turns on, the gate of the MOSFET is positive with
respect to the drain (-80V) and T₃ turns on. Turning T₃ on
connects the -80V battery to the SLIC through D₇. This in
turn reverse biases D₁₀, isolating the two supplies.
Transhybrid Balance (Voice Signal)
The purpose of the transhybrid circuit is to remove the
receive signal (V-REC) from the transmit signal (V-XMIT),
thereby preventing an echo on the transmit side. This is
accomplished by using an external op amp (usually part of
the CODEC) and by the inversion of the signal from the
SLIC’s 4-wire receive port (V_RX) to the SLIC’s 4-wire
transmit port (OUT1).
The external transhybrid circuit is shown in Figure 10. The
effects of capacitors C₅, C₇ and C₈ are negligible and
therefore omitted from the analysis. The input signal (V-
REC) will be subtracted from the output signal (V-XMIT) if I₁
equals I₂ are equal and opposite in phase. A node analysis
yields the following equation:
V_{-----–---RR----2-}E_-----C_{-- +} O_{------RU----3}T_-----1_{- = 0}
(EQ. 36)
The value of R₂ is then:
R₂ = –R₃ • ^V--_O---–---_UR----_T-^E----_1C---
(EQ. 37)
Given that OUT1 is equal to -1/3 of V-REC (Equation 16)
and V-REC is equal to VTR (A_{4-Wire-2-Wire} = 1, Equation
15), then R₂ = 3R_3. A transhybrid balance greater than 30dB
can be achieved by using 1% resistors values.
HC5517
-
+
180
PHASE
SHIFT
OF AC
SIGNAL
V_RX
C₈
R₉
-IN1
R₈
OUT1
EXTERNAL
TRANSHYBRID CIRCUIT
C₇
V-REC
C₅
INCOMING
AC TRANSMISSION
R₂
I₁
R₁
R₃
V-XMIT
-
+
I₂
OUTGOING
AC TRANSMISSION
SUMMING NODE CANCELS OUT
INCOMING AC TRANSMISSION FROM
OUT GOING TRANSMISSION
FIGURE 10. TRANSHYBRID CIRCUIT (VOICE SIGNAL)
Transhybrid Balance (Pulse Metering)
Transhybrid balance of the pulse metering signal is
accomplished in 2 stages. The first stage uses the SLIC’s
internal op amp to invert the phase of the pulse metering
signal. The second stage sums the inverted pulse metering
signal with the incoming signal for cancellation in the
transhybrid amplifier. A third network can be added to offset
both tip and ring by the peak amplitude of the pulse metering
signal. This will allow both the maximum voice and pulse
metering signals to occur at the same time with no distortion.
Pulse Metering
Pulse metering or Teletax is used outside the United States
for billing purposes at pay phones. A 12kHz or 16kHz burst
is injected into the 4-wire side of the SLIC and transmitted
across the tip and ring lines from the central office to the pay
phone. For more information about pulse metering than
covered here reference application note AN9608
“Implementing Pulse Metering for the HC5509 Series of
SLICs”.
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