hc5523 Intersil Corporation, hc5523 Datasheet - Page 12

• Manufacturer Part Number: hc5523
• Description: Lssgr/tr57 Co/loop Carrier Slic With Low Power Standby
• Manufacturer: Intersil Corporation

Before proceeding with an explanation of the loop current
detector, ground key detector and later the longitudinal
impedance, it is important to understand the difference
between a “metallic” and “longitudinal” loop currents. Figure 18
illustrates 3 different types of loop current encountered.
Case 1 illustrates the metallic loop current. The definition of
a metallic loop current is when equal currents flow out of tip
and into ring. Loop current is a metallic current.
Cases 2 and 3 illustrate the longitudinal loop current. The
definition of a longitudinal loop current is a common mode
current, that flows either out of or into tip and ring
simultaneously. Longitudinal currents in the on-hook state
result in equal currents flowing through the sense resistors R
and R
state result in unequal currents flowing through the sense
resistors R
flowing away from the SLIC, the current through R
metallic loop current plus the longitudinal current; whereas the
current through R
longitudinal current. Longitudinal currents are generated when
the phone line is influenced by magnetic fields (e.g., power
lines).
Loop Current Detector
Figure 18 shows a simplified schematic of the loop current
and ground key detectors. The loop current detector works
by sensing the metallic current flowing through resistors R
and R
transconductance amplifier (gm
of gm
Ground Key Detector
I
I
METALLIC
METALLIC
CASE 1
←
→
1
2
2
. This results in a current (I
and the metallic loop current. I
(Figure 18). And longitudinal currents in the off-hook
ORDERING INFORMATION
1
and R
I
I
LONGITUDINAL
LONGITUDINAL
CASE 2
2
2
. Notice that for case 2, longitudinal currents
is the metallic loop current minus the
←
←
12
I
I
LONGITUDINAL
LONGITUDINAL
1
CASE 3
) that is equal to the product
RD
→
→
FIGURE 18. LOOP CURRENT AND GROUND KEY DETECTORS
RING
) out of the
RD
TIP
then flows out the
HC5523
1
R
R
is the
1
2
1
+
+
-
-
1
HC5523
gm
gm
R
GROUND
KEY
COMPARATOR
R
equal to:
The I
compared to an internal 1.25V reference voltage. When the
voltage drop across R
configured for loop current detection, the DET pin goes low.
The hysteresis resistor R
effectively across R
threshold to be slightly higher than the off-hook to on-hook
threshold.
Taking into account the hysteresis voltage, the typical value
of R
Taking into account the hysteresis voltage, the typical value
of R
A filter capacitor (C
accuracy of the trip point in a noisy environment. The value
of this capacitor is calculated using the following Equation:
where: T = 0.5ms
I
R
R
C
H
RD
1
2
D
D
D
D
pin and through resistor R
=
=
=
D
D
=
RD
+
-
for the on-hook to off-hook condition is:
------------------------------------------------------------------------- -
I
for the off-hook to on-hook condition is:
------------------------------------------------------------------------- -
I
------- -
R
----------------------------------- -
ON HOOK to OFF
OFF HOOK to ON
T
I
D
TIP
current results in a voltage drop across R
COMPARATOR
gm
–
DIGITAL MULTIPLEXER
600
–
–
gm
1
I
(I
D
CURRENT
RING
I
GK
METALLIC
1
2
(I
TIP
LOOP
465
375
D
- I
D
=
I
) in parallel with R
D
RING
1
, causing the on-hook to off-hook
--------- -
300
D
2
)
I
L
exceeds 1.25V, and the logic is
–
–
)
H
HOOK
HOOK
adds an additional voltage
D
R
to V
H
+
-
+
1.25V
V
EE
REF
. The value of I
D
-
will improve the
R
DET
I
D
RD
-5V
D
R
V
EE
D
that is
(EQ. 24)
(EQ. 25)
(EQ. 26)
(EQ. 27)
RD
C
is
D