HC55171_03 Datasheet, PDF (12/20 Pages) Intersil Corporation – 5 REN Ringing SLIC for ISDN Modem/TA and WLL

HC55171
IR
=
–_----4_---(_---R_-----S----∆_----I_--L----)
R



R^-R----_R^-Z----0F--_{}
+
V_{-----R-----X--}
R
I_R
R
R
V_RX
1V_P
R
OUT1
+ ^∆IL - + ^∆IL -
R_P1
R_S1
TIP
A
-
+
-
+ ^VC
R/20
V_RING
R/2
VC
=
–4
RS

∆I _{L }
R^-R----_R^-Z----0F--_{}
+
VRX
†
+
-
2V_DC
I_OUT1 =
4R_S∆I_L
2R
R_Z0
R_RF
V_{TR RL}
∆V_IN
+
-
-
+R_S∆I_L
-
R_Z0
+
∆I_L
R_RF
+
R_P1 = R_P2 = R_S1 = R_S2 = R_S
+
-
2
V_TX
-IN₁
-
+
-
∆I_L
+
90kΩ 90kΩ
+
-
-R_S∆I_L
+
4R_-S∆I_L
-
4R_S∆I_L
+
R_Z0
R_RF
B
R_P2
RING
- ^∆IL +
R_S2
- ^∆IL +
-
+
+
- ^VD
VD
=
4RS

∆I _{L }
R^-R----_R^-Z----0F--_{}
– VRX
†
-
+
V_BAT
2
† GROUNDED FOR AC ANALYSIS
FIGURE 7. AC VOLTAGE GAIN AND IMPEDANCE MATCHING
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
∆V_IN
+
-
R_L
LOAD
SLIC
8RS_{R}-^R----_R-^Z----^0F--_{} + 4RS
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_IN
=
–8
R_S
∆I_L



R^-R----_R^-Z---0F--_{}
+ 4R_S∆I_L + R_L∆I_L
∆V_IN
=
∆I_L
–8
R_S



R^-R----_R^-Z---0F--_{}
+
4R_S
+
R_L
(EQ. 21)
(EQ. 22)
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:
8

R _{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)
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--^N--
∆I_L
=
–8

R _{S }
R^-R----_R^-Z---0F--_{}
+ [4R_S + R_L]
(EQ. 23)
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)
12