ISL5585 Datasheet, PDF (10/22 Pages) Intersil Corporation – 3.3V Ringing SLIC Family for Voice Over Broadband VOB

ISL5585
The gain of the Transmit Amplifier, set by R_S , determines
the programmed impedance of the device. The capacitor
C_FB blocks the DC component of the loop current. The
ground symbols in the model represent AC grounds, not
actual DC potentials.
The Sense Amplifier is configured as a 4 input differential
amplifier with a gain of 3/4. The voltage at the output of the
sense amplifier (V_SA) is calculated using superposition.
V_SA1 is the voltage resulting from V1, V_SA2 is the voltage
resulting from V2 and so on (reference Figure 4).
V_SA1
=
–³--
4
(
V₁
)
(EQ. 11)
V_SA2
=
3_--
4
(V₂)
(EQ. 12)
V_SA3
=
–³--
4
(
V₃
)
(EQ. 13)
V_SA4
=
3_--
4
(V₄)
(EQ. 14)
V_SA
=
[(
V₂
–
V₁
)
+
(V₄
–
V₃
)
]
3_--
4
=
[∆V
+
∆V]
3_--
4
(EQ. 15)
Where ∆V is equal to I_MR_SENSE (R_SENSE = 20Ω)
V_SA
=
2
(
∆I_M
×
20)
3_--
4
=
∆I _M 30
(EQ. 16)
The voltage at VTX is equal to:
V_TX
=
–V_I
N



R^--R---_I^-S-_N^--_{}
–V_S
A


-^R8----_k^S--_
=
–V_IN



R^--R---_I^-S-_N^--_{}
–(
∆I_M
30)


-^R8----_k^S--_
(EQ. 17)
V_TR is defined in Figure 4, note polarity assigned to V_TR:
V_TR
=
2(V_TX)
=
2

 ^{V I N}



R^--R---_I^-S-_N^--_{}
+
(
∆I_M 30 )


-^R8----_k^S--_



(EQ. 18)
Setting V_IN equal to zero in EQ 18, defining Z_O = -V_TR/∆I_M
and substituting it into EQ18 will enable the user to
determine the required feedback to match the line
impedance at V_2W as shown in Equation 19.
Z_O
=
--------¹---------
133.33
R_S
(EQ. 19)
2-Wire Impedance Matching
Z_O is the source impedance of the device and is defined as.
Z_O = Z_L – 2R_P
(EQ. 20)
Z_L is the line impedance and R_P is the external protection
resistor. R_S is defined as:
R_S = 133.33(Z_L – 2R_P)
(EQ. 21)
I_X
-
+
INTERSIL
ISL5585
+
V_2W
-
TIP
- Z_L
I_M
+ ^IM _-
V₂
R_P
Z₀
-
R
R_{SENSE V1}
-
+
20Ω
+ ^IM _-
FEED
AMPLIFIER
+
V_TR
+
-E^G
- ^IM +
RING ^RP
FEED
R_SENSE AMPLIFIER
V₃
V₄
+
20Ω
- ^IM ⁺
-
I_X
+
-
R
I_X ^{RECEIVE BLOCK} _IX
R
1:1
R
FEEDBACK
TA
AMPLIFIER
4R
3R
4R
-
8k
4R
+
4R
3R
V_SA = ∆I_M30
SENSE
AMPLIFIER
AUX
V_TX
+
V_TX
-
R_S
-IN
C_FB
VFB
FIGURE 4. AC SIGNAL TRANSMISSION MODEL
R_IN
V_IN
FROM CODEC
10