HC5517_00 Datasheet, PDF (8/19 Pages) Intersil Corporation – 3 REN Ringing SLIC For ISDN Modem/TA and WLL

HC5517
Substituting the expressions for V_C and V_D :
2
×



–
4
R
S

∆I_{L}
R^-R----89-_{}
+

V_{R X}
∆I_L
=
--------------------------------------------------------------------------
R_L + R₁₁ + R₁₂ + R₁₃ + R₁₄
(EQ. 12)
Equation 12 simplifies to:
∆I_L = -²---^V-----^R----^X---₈--–-₀---⁴-₀---⁰---⁰----^∆----^I-^L--
(EQ. 13)
Solving for ∆I_L results in:
∆I_L = ^V-₆----^R₀---₀-^X--
(EQ. 14)
Equation 14 is the loop current with respect to the feedback
network. From this, the 4-wire to 2-wire and the 2-wire to
4-wire AC voltage gains are calculated. Equation 15 shows
the 4-wire to 2-wire AC voltage gain is equal to one.
A_{4W – 2W} = _V-^V----_RT----^RX-- = ^∆----^I-_V-^L---_R-⁽--^R--_X--^L----⁾ = ^V--⁶--------^0R------^0V--X-----_R-⁽--⁶--_X--⁰---⁰----⁾ = 1
(EQ. 15)
Equation 16 shows the 2-wire to 4-wire AC voltage gain is
equal to negative one-third.
A_{2W – 4W}
=
V_{-----VO----TU---R--T---1-}
=
–4
R_S
∆I_L



RR_{-----89-}
---------_∆----_I-_L----₍--_R-----_L---₎---------
=
–----2_-V-6----0_--0-R---0_----0--X---^V---_6---(----0^R₆------₀--₀^X-----_0-(_--1_-)---)
=
–¹³--
(EQ. 16)
Impedance Matching
The feedback network, described above, is capable of
synthesizing both resistive and complex loads. Matching the
SLIC’s 2-wire impedance to the load is important to
maximize power transfer and minimize the 2-wire return
loss. The 2-wire return loss is a measure of the similarity of
the impedance of a transmission line (tip and ring) and the
impedance at it’s termination. It is a ratio, expressed in
decibels, of the power of the outgoing signal to the power of
the signal reflected back from an impedance discontinuity.
Requirements for Impedance Matching
Impedance matching of the HC5517 application circuit to the
transmission line requires that the impedance be matched to
points “A” and “B” in Figure 3. To do this, the sense resistors
R₁₁, R₁₂ , R₁₃ and R₁₄ must be accounted for by the
feedback network to make it appear as if the output of the tip
and ring amplifiers are at points “A” and “B”. The feedback
network takes a voltage that is equal to the voltage drop
across the sense resistors and feeds it into the summing node
of the tip amplifier. The effect of this is to cause the tip feed
voltage to become more negative by a value that is
proportional to the voltage drop across the sense resistors
R₁₁ and R₁₃. At the same time the ring amplifier becomes
more positive by the same amount to account for resistors
R₁₂ and R₁₄ .
The net effect cancels out the voltage drop across the feed
resistors. By nullifying the effects of the feed resistors the
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):
–4
R
S
∆I

L^

R^-R----89-_{}
+
2 R_S ∆I_L – ∆V _{I N}
+
R_L∆I_L
+

2R_S∆I_L–4R_S∆I_L

RR_{-----89- }
=
0
(EQ. 17)
∆V_IN
=
–8
R_S
∆I_L



RR_{-----89-}
+ 4R_S∆I_L + R_L∆I_L
∆V_IN
=
∆I_L
–8
R_S



RR_{-----89-}
+
4R_S
+
R_L
(EQ. 18)
(EQ. 19)
Equation 19 can be separated into two terms, the feedback
(-8R_S(R₈/R₉)) and the loop impedance (+4R_S+R_L).
∆_---∆-V_---I--IL--N-
=
–8
R_S



RR_{-----89- }
+ 4R_S + R_L
(EQ. 20)
The result is shown in Equation 20. Figure 4 is a schematic
representation of Equation 15.
R_L
∆V_IN
+
-
LOAD
SLIC
8
R
S



RR_{-----89--}
+
4RS
FIGURE 4. SCHEMATIC REPRESENTATION OF EQUATION 20
To match the impedance of the SLIC to the impedance of
the load, set:
R_L
=
8
R_S



RR_{-----89-}
+ 4R_S
(EQ. 21)
If R₉ is made to equal 8R_S then:
R_L = R₈ + 4R_S
(EQ. 22)
Therefore to match the HC5517, with R_S equal to 50Ω, to a
600Ω load:
R₉ = 8R_S = 8(50Ω) = 400Ω
(EQ. 23)
and:
R₈ = R_L–4R_S = 600Ω – 200Ω = 400Ω
(EQ. 24)
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