HC5517 Intersil Corporation, HC5517 Datasheet - Page 15

• Manufacturer Part Number: HC5517
• Description: 3 REN Ringing SLIC For ISDN Modem/TA and WLL
• Manufacturer: Intersil Corporation

Additional Tip and Ring Offset Voltage
A DC offset is required to level shift tip and ring from ground
and V
4V below ground and the ring amplifier is offset 4V above
V
signal could pass through the SLIC without distortion. There-
fore, to maintain distortion free transmission of pulse meter-
ing and voice, an additional offset equal to the peak of the
pulse metering signal is required.
The tip and ring voltages are offset by a voltage divider
network on the V
designed as the 4-wire side voice input for the SLIC.
Figure 13 details the circuit used to generate the additional
offset voltage.
The amplifier shown is the tip amplifier. Other signals are con-
nected to the summing node of the amplifier but only those
components used for the offset generation are shown. The off-
set generated at the output of the tip amplifier is summed at the
ring amplifier inverting input to provide a positive offset from the
battery voltage. The connection to the ring amplifier was omit-
ted from Figure 13 for clarity, refer to Figure 3 for details.
The term V
pulse metering signal. The value of the offset voltage is cal-
culated as the peak value of the pulse metering signal.
Equation 45 assumes the amplitude of the pulse metering
signal is expressed as an RMS voltage.
The value of R
equation:
The component labeled R is the internal summing resistor of
the tip amplifier and has a typical value of 108k . The value
of R
Staying within these limits will minimize the parallel loading
effects of the internal resistor R on R
the constant power dissipation introduced by the divider.
Solving Equation 45 for 1V
requirement for V
and substituting the values for V
R
value without significantly changing the offset voltage.
R
V
2-WIRE SIDE
BAT
6
6
PMO
. The value of R
=
FIGURE 13. PULSE METERING OFFSET GENERATION
7
. The 4V offset was designed so that the peak voice
BAT
should be selected in the range of 4.99k
----------------- -
R
=
R
7
7
+
R
respectively. By design, the tip amplifier is offset
2 V
R
PMO
R
+
-
5 V
------------------------- -
PM
–
V
is defined to be the offset required for the
6
RX
PMO
PMO
6
PMO
can be calculated from the following
can be rounded to the nearest standard
pin. The V
R
. Setting R
V
PMO
V
RX
74
RMS
PMO
RX
+5V
7
TRANSHYBRID AMP
TO VOICE INPUT OF
R
R
pin is a unity gain input
of Equation 46 to 10k
6
7
and R yields 23.2k for
results in a 1.414V
7
C
7
as well as minimize
C
5
4-WIRE SIDE
VOICE INPUT
and 10k .
(EQ. 45)
(EQ. 46)
HC5517
Single Low Voltage Supply Operation
The application circuit shown Figure 15 requires 2 low
voltage supplies (+5V, -5V). The following application offers
away to make use of a 2.5V reference, provided with some
CODEC, to operate the transhybrid balance amplifier from
a single +5V supply. The implementation is shown in
Figure 14. Notice that the three inputs from the SLIC must all
be AC coupled to insure the proper DC gain through the
CODECs internal op amp. The resistor Ra is not used for
gain setting and is only intended to balance the DC offsets
generated by the input bias current of the CODEC amplifier.
If the DC offsets generated by the input bias currents are
negligible, then Ra may be omitted from the circuit. Ca may
be required for decoupling of the voltage reference pin and
does not contribute to the response of the amplifier.
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:
For multi layer boards, the traces connected to tip should not
cross the traces connected to ring. Since they will be carrying
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
input and the transmit path is any trace associated with V
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
signals are nearly static, care should be taken to minimize
coupling of the sharp digital edges to the analog signals.
FIGURE 14. SINGLE LOW VOLTAGE SUPPLY OPERATION
• V
• -IN1 pin 13, the inverting input of the internal amplifier.
• V
• V
• U1 pin 2, inverting input of external amplifier.
OUT
RX
REF
RING
V
V
PM
RX
1
pin 27, the 4-wire voice input.
pin 3, the noninverting input to ring feed amplifier.
0.1 F
0.1 F
0.1 F
pin 24, the 20V/V input for the ringing signal
R
R
R
2
3
4
24.9k
24.9k
R
0.1 F
1
Ca
Ra
-
+
CODEC
2.4V
REF
RX
TX