HC5503 Intersil Corporation, HC5503 Datasheet - Page 5

• Manufacturer Part Number: HC5503
• Description: Low Cost 24V SLIC For PABX/Key Systems
• Manufacturer: Intersil Corporation

Design Information
Line Feed Amplifiers
The line feed amplifiers are high power operational
amplifiers and are connected to the subscriber loop through
150 of feed resistance as shown in Figure 1. The feed
resistors and synthesized impedance via feedback provide a
600 balanced load for the 2-wire to 4-wire transmission.
The tip feed amplifier is configured as a unity gain
noninverting buffer. A -4V bias (derived from the negative
battery (V
the amplifier. Hence, the tip feed DC level is at -4V. The
principal reason for this offset is to accommodate sourcing
and sinking of longitudinal noise currents up to 15mA
without saturating the amplifier output and to provide
sufficient overhead for receive signals. The tip feed amplifier
also feeds the ring feed amplifier, which is configured as a
unity gain inverting amplifier as seen from the tip feed
amplifier. The noninverting input to the ring feed amp is
biased at a V
has a noninverting gain of 2. Thus, the DC output at ring
feed is:
V
For a -24V battery, V
feed across the loop provided by the SLIC is 16V. When the
subscriber goes off-hook this DC feed causes current
(metallic current) to flow around the loop.
NOTE: Grounded for AC analysis.
RF
(DC) = (4 + V
Z
R
BAT
IN
B1
= R
BAT
) in the bias network) is applied to the input of
RING
B2
TIP
/2. Looking into this terminal the amplifier
BAT
= R
RF
S
) Volts
-
150
150
+
= 150
R
R
B2
I
= -20V. Hence, the nominal battery
L
B1
I
L
+
-
5
RING FEED
TIP FEED
FIGURE 1. IMPEDANCE MATCHING CIRCUITRY
R
+
-
Va
+
-
+
-
R
RMS
V
BAT
2
90k
HC5503
+
+
-
-
+ 4V
-
+4RS I
(NOTE)
HC5503
DC
(NOTE)
The received audio signal R
and appears at the tip feed terminal. It is also fed through the
ring feed amplifier and is inverted. Thus, a differential signal of
2V
causes AC audio currents to flow around the loop which are
then AC coupled to the earpiece of the telephone set.
2-Wire Impedance Matching
The HC5503 is optimized for operation with a -24V battery.
Impedance matching to a 600 load, is achieved through
the combination of the feed resistors (R
negative feedback through resistor R
R
and provide negative feedback to synthesize the remaining
300 required to match a 600 line.
The impedance looking into the tip terminal is 150 (R
plus the synthesized impedance of the tip amplifier. The
synthesized tip impedance is equal to the tip feed voltage Va
divided by IL. (Note, the tip feed amplifier is a voltage
follower. Thus, the tip feed voltage is equal to the receive
input voltage V
impedance of the ring terminal is calculated the same way
and is the ring feed voltage divided by IL. (Note, the ring
feed voltage is equal in magnitude to the tip feed voltage, but
opposite in phase as a result of the ring feed amplifier gain.)
L
B1
RX
+ 2
-
RX
and R
appears between tip feed and ring feed. The R
4RS I
B2
+
TX
C
L
3
are sense resistors that detect loop current
RX
, both are labeled Va.) The synthesized
R
3
C
4
TO TRANSHYBRID OP-AMP
R
R
2
1
X
is fed into the tip feed amplifier
V
IN
2
(reference Figure 1).
B1
INPUT
FROM
CODEC
, R
B2
) and
X
signal
B1
)