HC55120_06 Datasheet, PDF (17/36 Pages) Intersil Corporation – Low Power Universal SLIC Family

HC55120, HC55121, HC55130, HC55140, HC55142, HC55143, HC55150
example R_D is calculated to be 41.6kΩ (500/12mA). The
next closest standard value is 41.2kΩ.
R_D
=
--⁵---⁰----⁰---
I_SHD
(EQ. 7)
The true value of ISH-, for the selected value of R_D is given
by Equation 8:
ISH-
=
5_----0_---0_--
R_D
(0.6)
(EQ. 8)
For the example above, ISH- equals 7.28mA (500 x 0.6/
41.2K). Verify that the value of ISH- is above the suspected
line leakage of the application. The UniSLIC family will
provide a constant on hook voltage level for leakage currents
up to this value of line leakage.
DC FEED CURVE
V_BH
V_SAT
2.5V
V_OH(off)
OFF HOOK
OVER HEAD
I_OH
ISH-
I_LOOP(min)
LOOP CURRENT
The R_OH resistor, which
is used to set the offhook
overhead voltage, is
calculated using
Equations 9 and 10.
I_OH is defined as the
difference between the
I_LOOP(min) and ISH-.
Substituting Equation 8
for ISH- into Equation 9 and solving for R_OH defines R_OH in
terms of I_LOOP(min) and R_D.
R_OH
= -⁵---⁰---⁰-- = ------------------⁵---⁰---⁰-------------------
I_OH I_LOOP(min)- ISH-
(EQ. 9)
Equation 10 can be used to determine the actual ISH- value
resulting from the R_D resistor selected. The value of R_D
should be the next standard value that is lower than that
calculated. This will insure meeting the I_LOOP(min)
requirement. ROH for the above example equals 39.1kΩ.
R_OH
=
---------------------^R-----^D----⁵---⁰----⁰----------------------
R_DI_LOOP(min)- 500(.6)
(EQ. 10)
The current limit is set by a single resistor and is calculated
using Equation 11.
R_LIM
=
---------¹---⁰---⁰----⁰----------
I_LOOP(max)
(EQ. 11)
DC FEED CURVE
The maximum loop
V_BH
V_SAT
2.5V
V_OH(off)
resistance is calculated
using Equation 12. The
resistance of the
protection resistors
R LOOP(MAX)
(2R_P) is subtracted out
to obtain the maximum
I_LOOP(min)
LOOP CURRENT
loop length to meet the
required off hook
overhead voltage. If R_LOOP(MAX) meets the loop length
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.
R_LOOP(max)
=
V_{-----B---H-----–---}[_--V_{----S----A----T-----+-----}2_----V_-----+-----V_{----O-----H----(--o---f--f--)---}]
I_LOOP(min)
- 2R _P
(EQ. 12)
SLIC in the Active Mode
Figure 17 shows a simplified AC transmission model. Circuit
analysis yields the following design equations:
V_A
=
I_M
×
2R_S
×
----¹-----
80 k
×
200 ( Z _{T R}
–
2R _P )
×
5
(EQ. 13)
V_A
=
--^I--^M---
2
(Z_TR
–
2R_P
)
Node Equation
-^V-----^R---^X---
500k
-
---^V-----^A----
50 0k
=
I_X
Substitute Equation 14 into Equation 15
I_X
=
-^V----^R----^X---
500k
-
--^I--^M-----⁽--^Z----^T----^R-----–-----²---^R-----^P----⁾
100 0k
(EQ. 14)
(EQ. 15)
(EQ. 16)
Loop Equation
I_X500k - V_TX′ + I_X500k = 0
Substitute Equation 16 into Equation 17
V_TX′ = 2V_RX – I_M(Z_TR – 2R_P)
Loop Equation
V_TR-I_M2R_P + V_TX′ = 0
Substitute Equation 18 into Equation 19
V_TR = I_MZ_TR – 2V_RX
(EQ. 17)
(EQ. 18)
(EQ. 19)
(EQ. 20)
Substituting -V_TR/Z_L into Equation 20 for I_M and rearranging
to solve for V_TR results in Equation 21
⎛
V _{TR ⎝⎜}
1
+
-^Z--_Z-^T---_L-^R--_{⎠⎟⎞}
=
– 2 V _RX
(EQ. 21)
where:
V_RX = The input voltage at the VRX pin.
V_A = An internal node voltage that is a function of the loop
current detector and the impedance matching networks.
I_X = Internal current in the SLIC that is the difference
between the input receive current and the feedback current.
I_M = The AC metallic current.
R_P = A protection resistor (typical 30Ω).
Z_T = An external resistor/network for matching the line
impedance.
´ V_TX = The tip to ring voltage at the output pins of the SLIC.
17
FN4659.13
June 1, 2006