HC55120 Intersil Corporation, HC55120 Datasheet - Page 16

• Manufacturer Part Number: HC55120
• Description: Low Power Universal SLIC Family
• Manufacturer: Intersil Corporation

for ISH- into Equation 9 and solving for R
terms of I
Equation 10 can be used to determine the actual ISH- value
resulting from the R
should be the next standard value that is lower than that
calculated. This will insure meeting the I
requirement. ROH for the above example equals 39.1k
The current limit is set by a single resistor and is calculated
using Equation 11.
overhead voltage. If R
requirements you are done. If the loop length needs to be
longer, then consider adjusting one of the following: 1) the
SHD threshold, 2) minimum loop current requirement or 3)
the on and off hook signal levels.
SLIC in the Active Mode
Figure 17 shows a simplified AC transmission model. Circuit
analysis yields the following design equations:
HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151
R
R
R
R
V
V
A
A
OH
OH
LIM
LOOP(max)
V
= I
SAT
=
V
V
OH(off)
BH
=
=
V
=
M
------- Z
I
DC FEED CURVE
DC FEED CURVE
2
SAT
M
-----------------------------
I
----------------------------------------------------------- -
R
V
LOOP(max)
500
--------- -
I
OH
BH
D
2R
LOOP(min)
1000
I
ISH-
TR
LOOP(min)
=
2.5V
S
=
LOOP CURRENT
LOOP CURRENT
V
------------------------------------------------------------------------------ - -2R
–
R
------------------------------------------- -
I
BH
2.5V
LOOP(min)
2R
--------- -
80k
D
I
OH
1
500
–
P
I
I
LOOP(min)
LOOP(min)
V
D
- 500(.6)
and R
500
200 Z
SAT
resistor selected. The value of R
LOOP(MAX)
I
OFF HOOK
OVER HEAD
V
LOOP(min)
OH(off)
- ISH-
+
D
TR
4-16
2V
.
–
+
2R
V
OH off
P
meets the loop length
The R
is used to set the offhook
overhead voltage, is
calculated using
Equations 9 and 10.
I
difference between the
I
Substituting Equation 8
The maximum loop
resistance is calculated
using Equation 12. The
resistance of the
protection resistors
(2R
to obtain the maximum
loop length to meet the
required off hook
OH
LOOP(min)
5
P
LOOP(min)
is defined as the
OH
) is subtracted out
OH
defines R
P
resistor, which
and ISH-.
(EQ. 10)
(EQ. 11)
(EQ. 12)
(EQ. 13)
(EQ. 14)
(EQ. 9)
OH
D
in
Substitute Equation 14 into Equation 15
I
Substitute Equation 16 into Equation 17
Substitute Equation 18 into Equation 19
Substituting -V
to solve for V
where:
V
V
current detector and the impedance matching networks.
I
between the input receive current and the feedback current.
I
R
Z
impedance.
V
V
protection resistors.
Z
Z
protection resistors.
(AC) 4-Wire to 2-Wire Gain
The 4-wire to 2-wire gain is equal to V
From Equation 21 and the relationship Z
Notice that the phase of the 4-wire to 2-wire signal is 180
out of phase with the input signal.
Loop Equation
V
V
Node Equation
------------ - -
500k
Loop Equation
I
V
V
G
V
X
X
M
X
A
T
L
TR
RX
TX
TR
TX
TR
TR
TR
P
RX
4-2
500k - V
= Internal current in the SLIC that is the difference
=
= The AC metallic current.
= The line impedance.
= An external resistor/network for matching the line
= An internal node voltage that is a function of the loop
= A protection resistor (typical 30 ).
-I
´
= The tip to ring voltage including the voltage across the
= The input impedance of the SLIC including the
=
------------ - -
500k
1
= The input voltage at the VRX pin.
V
=
= The tip to ring voltage at the output pins of the SLIC.
M
=
RX
+
------------ - = I
500k
2R
V
---------- - = -2
V
I
V
2V
M
Z
---------- -
RX
TR
Z
A
TR
P
Z
TX
L
RX
TR
-----------------------------------------
+ V
I
M
TR
–
+ I
–
=
X
TR
TX
Z
I
2V
-------------------------
Z
1000k
X
M
– V
results in Equation 21
TR
L
/Z
2
500k = 0
RX
Z
+ Z
= 0
Z
L
–
TR
RX
L
2R
into Equation 20 for I
TR
–
P
2R
=
P
–
2
--------------------------------------------- -
Z
L
+
--------- -
200
Z
Z
T
L
TR
+
T
/V
2R
M
= 200(Z
RX
P
and rearranging
.
TR
(EQ. 16)
(EQ. 22)
-2R
(EQ. 15)
(EQ. 17)
(EQ. 18)
(EQ. 19)
(EQ. 20)
(EQ. 21)
P
o
).