HC5515 Datasheet, PDF (12/17 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5515
R_RT
C_RT
R₃
TIP
R₄
E_RG
R_{1 DT}
-
+
DR
R₂
RING TRIP
COMPARATOR
DET
RING
RING
RELAY
V_BAT
RINGRLY
HC5515
FIGURE 18. RING TRIP CIRCUIT FOR BATTERY BACKED
RINGING
Longitudinal Impedance
The feedback loop described in Figure 19(A, B) realizes the
desired longitudinal impedances from tip to ground and from
ring to ground. Nominal longitudinal impedance is resistive
and in the order of 22Ω.
In the presence of longitudinal currents this circuit
attenuates the voltages that would otherwise appear at the
tip and ring terminals, to levels well within the common mode
range of the SLIC. In fact, longitudinal currents may exceed
the programmed DC loop current without disturbing the
SLIC’s VF transmission capabilities.
The function of this circuit is to maintain the tip and ring
voltages symmetrically around V_BAT/2, in the presence of
longitudinal currents. The differential transconductance
amplifiers G_T and G_R accomplish this by sourcing or sinking
the required current to maintain V_C at V_BAT/2.
When a longitudinal current is injected onto the tip and ring
inputs, the voltage at VC moves from it’s equilibrium value
VBAT/2. When VC changes by the amount DVC, this change
appears between the input terminals of the differential
transconductance amplifiers GT and GR. The output of GT
and GR are the differential currents DI1 and DI2, which in
turn feed the differential inputs of current sources IT and IR
respectively. IT and IR have current gains of 250 single
ended and 500 differentially, thus leading to a change in IT
and IR that is equal to 500(DI) and 500(DI2).
The circuit shown in Figure 19(B) illustrates the tip side of
the longitudinal network. The advantages of a differential
input current source are: improved noise since the noise due
to current source 2I_O is now correlated, power savings due
to differential current gain and minimized offset error at the
Operational Amplifier inputs via the two 5kΩ resistors.
Digital Logic Inputs
Table 1 is the logic truth table for the TTL compatible logic
input pins. The HC5515 has an enable input pin (E0) and
two control inputs pins (C1, C2).
The enable pin E0 is used to enable or disable the DET
output pin. The DET pin is enabled if E0 is at a logic level 0
and disabled if E0 is at a logic level 1.
A combination of the control pins C1 and C2 is used to
select 1 of the 4 possible operating states. A description of
each operating state and the control logic follow:
Open Circuit State (C1 = 0, C2 = 0)
In this state the SLIC is effectively off. All detectors and
both the tip and ring line drive amplifiers are powered
down, presenting a high impedance to the line. Power
dissipation is at a minimum.
Active State (C1 = 0, C2 = 1)
The tip output is capable of sourcing loop current and for
open circuit conditions is about -4V from ground. The ring
output is capable of sinking loop current and for open circuit
conditions is about V_BAT +4V. VF signal transmission is
normal. The loop current detector is active, E0 determines if
the detector is gated to the DET output.
I_LONG
I_LONG
TIP
+
∆V_T-
R_LARGE
I_T
∆I₁
∆I₁
G_T
I_LONG
V_C ⁺
-
I_LONG
R_LARGE
+ ^RING
∆V_R-
HC5515
V_BAT/2
G_R
∆I₂
∆I₂
I_R
TIP
R_LARGE
V_C
R_LARGE
RING
FIGURE 19A.
FIGURE 19. LONGITUDINAL IMPEDANCE NETWORK
TIP CURRENT SOURCE
WITH DIFFERENTIAL INPUTS
20Ω
5kΩ ^5kΩ
+^-
∆I₁
∆I₁
V_BAT/2
2I₀
TIP DIFFERENTIAL
TRANSCONDUCTANCE
AMPLIFIER
FIGURE 19B.
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