HC55171 Datasheet, PDF (10/18 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
feedback circuitry becomes relatively easy to match the
impedance at points “A” and “B”.
Impedance Matching Design Equations
Matching the impedance of the SLIC to the load is
accomplished by writing a loop equation starting at V_D and
going around the loop to V_C.
The loop equation to match the impedance of any load is as
follows (note: V_RX = 0 for this analysis):
–4R_S
∆I_L



R^-R----_R^-Z---0F--_{}
+
2 R_S ∆I_L –∆V_{I N}
+
R_L ∆I_L
+
2
R_S
∆I_L
–4
R_S
∆I_L



R^-R----_R^-Z---0F--_{}
(EQ. 20)
∆V _{I N}
=
–8R_S
∆I_L



R^-R----_R^-Z---0F--_{}
+ 4R_S∆I_L + R_L∆I_L
∆V _{I N}
=
∆I_L
–8

R_{S }
R^-R----_R^-Z---0F--_{}
+ 4R_S + R_L
(EQ. 21)
(EQ. 22)
Equation 22 can be separated into two terms, the feedback
(-8R_S(R_Z0/R_RF)) and the loop impedance (+4R_S+R_L).
-^∆--_∆-^V---_I-^I-_L-^N--
=
–8
R_S



R^-R----_R^-Z---0F--_{}
+ [4R_S + R_L]
(EQ. 23)
IR = ^–----⁴----⁽--^R---_R-^S-----^∆----^I-^L-----^{)}_R-^R----_R-^Z----^0F--_{} + ^V-----^RR-----^X--
I_R
R
R
R
+ ^∆IL - + ^∆IL -
R/20
R_P1
R_S1
TIP
A
-
+
-
+ ^VC
R/2
VC = –4RS∆
-
V_{TR RL}
∆I_L
+
R_P1 = R_P2 = R_S1 = R_S2 = R_S
∆V_IN
+
-
-
∆I_L
+
B
R_P2
RING
- ^∆IL +
R_S2
- ^∆IL +
90kΩ 90kΩ
-
+
+
- ^VD
VD = 4RS
FIGURE 7. AC VOLTAGE GAI
∆V_IN
+
-
R_L
LOAD
SLIC
8
R
S



R^-R----_R^-Z----0F--_{}
+
4RS
FIGURE 8. SCHEMATIC REPRESENTATION OF EQUATION 20
The result is shown in Equation 23. Figure 8 is a schematic
representation of Equation 18. To match the impedance of
the SLIC to the impedance of the load, set:
8R_S



R^-R----_R^-Z---0F--_{}
+
4R_S
=
R_L
(EQ. 24)
If R_RF is made to equal 8R_S then:
R_Z0 + 4R_S = R_L
(EQ. 25)
Therefore to match the HC5517, with R_S equal to 50Ω, to a
600Ω load:
R_RF = 8R_S = 8(50Ω) = 400Ω
(EQ. 26)
and
R_Z0 = R_L–4R_S = 600Ω – 200Ω = 400Ω
(EQ. 27)
To prevent loading of the V_TX output, the value of R_Z0 and
R_RF are typically scaled by a factor of 100:
KR_Z0 = 40kΩ
KR_RF = 40kΩ
(EQ. 28)
Since the impedance matching is a function of the voltage
gain, scaling of the resistors to achieve a standard value is
recommended.
For complex impedances the above analysis is the same.
(KR_RF
=
40 k Ω )


K
R_Z
0
=
100 ( R e s i s t i v e
–
200 )
+
R_-----e_----1a---0-c_---0t---i_-v_----e_{- }
(EQ. 29)
Refer to application note AN9607 (“Impedance Matching
Design Equations for the HC5509 Series of SLICs”) for the
values of KR_RF and KR_Z0 for many worldwide typical line
impedances.
Through SLIC Ringing
The HC55171 uses linear amplification to produce the ringing
signal. As a result the ringing SLIC can produce sinusoid,
trapezoid or square wave ringing signals. Regardless of the
wave shape, the ringing signal is balanced. The balanced
waveform is another way of saying that the tip and ring DC
potentials are the same during ringing. The following figure
shows the Tip and Ring waveforms for sinusoid and trapezoid
wave shapes as can be displayed using an oscilloscope.
Pertinent Bellcore Ringing Specifications
Bellcore has defined bounds around the existing unbalanced
ringing signal that is supplied by the central office. The
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