HC55171 Datasheet, PDF (15/18 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
Polarity Reversal
The HC55171 supports applications that use polarity reversal
outside the speech phase of a call connection. The most com-
mon implementation of this type of polarity reversal is used
with pay phones. By reversing the polarity of the tip and ring
terminals of a pay phone, DC current changes direction in a
solenoid and the coins are released from the phone. To
reverse the polarity of the HC55171, simply toggle the V_RING
input high. Setting the V_RING input high will cause Tip and
Ring to reverse polarity.
Transhybrid Balance
Since the receive signal and its echo are 180 degrees out of
phase, the summing node of an operational amplifier can be
used to cancel the echo. Nearly all CODECs have an inter-
nal amplifier for echo cancellation. The following Figure 19
shows the cancellation amplifier circuit.
R_A
R_F
V_RX
R_B
V_OUT1
-
+
VO
FIGURE 19. TRANHYBRID AMPLIFIER CIRCUIT
When the SLIC is matched to a 600Ω load, the echo ampli-
tude is 1/3 the receive input amplitude. Therefore, by config-
uring the transhybrid amplifier with a gain of 3 in the echo
path, cancellation can be achieved. The following equations:
V_O
=

– _ V _R
X



R^-R----_AF^--_{}
+
V_O
U
T1



R^-R----_BF^--_{}



(EQ. 37)
Substituting the fact that V_OUT1 is -1/3 of V_RX
V_O
=

– _ V _R
X



R^-R----_AF^--_{}
–
V_R
X


13_{--}



R^-R----_BF^--_{}



(EQ. 38)
Since cancellation implies that under these conditions, the
output V_O should be zero, set Equation 37 equal to zero and
solve for R_B.
R_B = ^R---₃--^A--
(EQ. 39)
Another outcome of the transhybrid gain selection is the 2-
wire to 4-wire gain of the SLIC as seen by the CODEC. The
1/3 voltage gain in the transmit path is relevant to the receive
input as well as any signals from the 2-wire side. Therefore
by setting the V_OUT1 gain to three in the previous analysis,
the 2-wire to 4-wire gain was set to unity.
Single Supply Codec Interface
The majority of CODECs that interface to the ringing SLIC
operate from a single +5V supply and ground. Figure 20
shows the circuitry required to properly interface the ringing
SLIC to the single supply CODEC.
The CODEC signal names may vary from different
manufacturers, but the function provided will be the same.
The DC reference from the CODEC is used to bias the
analog signals between +5V and ground. The capacitors are
required so that the DC gain is unity for proper biasing from
the CODEC reference. Also, the capacitors block DC signals
that may interfere with SLIC or CODEC operation.
Layout Guidelines and Considerations
The printed circuit board trace length to all high impedance
nodes should be kept as short as possible. Minimizing length
will reduce the risk of noise or other unwanted signal pickup.
The short lead length also applies to all high gain inputs. The
set of circuit nodes that can be categorized as such are:
• V_RX pin 27, the 4-wire voice input.
• -IN1 pin 13, the inverting input of the internal amplifier.
• V_REF pin 3, the noninverting input to ring feed amplifier.
• V_RING pin 24, the 20V/V input for the ringing signal.
For multi layer boards, the traces connected to tip should not
cross the traces connected to ring. Since they will be carry-
ing high voltages, and could be subject to lightning or surge
depending on the application, using a larger than minimum
trace width is advised.
The 4-wire transmit and receive signal paths should not
cross. The receive path is any trace associated with the V_RX
input and the transmit path is any trace associated with V_TX
output. The physical distance between the two signal paths
should be maximized to reduce crosstalk.
The mode control signals and detector outputs should be
routed away from the analog circuitry. Though the digital sig-
nals are nearly static, care should be taken to minimize cou-
pling of the sharp digital edges to the analog signals.
The part has two ground pins, one is labeled AGND and the
other BGND. Both pins should be connected together as
close as possible to the SLIC. If a ground plane is available,
then both AGND and BGND should be connected directly to
the ground plane.
A ground plane that provides a low impedance return path
for the supply currents should be used. A ground plane pro-
vides isolation between analog and digital signals. If the lay-
out density does not accommodate a ground plane, a single
point grounding scheme should be used.
V_RX
VOUT1
HC55171
R_A
R_F
R_B
CODEC
RX OUT
-
+
TX IN
+2.5V
+
-
FIGURE 20. SINGLE SUPPLY CODEC INTERFACE
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