HC5517 Datasheet, PDF (10/18 Pages) Intersil Corporation – 3 REN Ringing SLIC For ISDN Modem/TA and WLL

HC5517
R₁₉ = ⁽---^V----^C----^C------–₍---_V-^V---_C-^D----+-¹---³-_V---–--_D---^V-₆---^D₎---_R-⁶---_I-–-_N----^V-+---^C--_V--⁾---_C⁽--^R--_R---¹-₁--⁸-₈---^•-----^R----^I--^N-----⁾
where: V_D13 forward drop of D₁₃, 0.63V.
(EQ. 30)
V_D6 forward drop of D₆, 0.54V.
R₁₈ is the shunt resistor of the divider, 1.1kΩ.
R_IN is the input impedance of V_RING, 5.4kΩ.
V_C is the required centering voltage, 1.8V, V_BAT = -80V.
V_CC is the +5V supply.
Centering Voltage Logic Control
The pnp transistor T₂ is used to defeat the voltage divider
formed by R₁₉, R₁₈, D₁₃ and D₆. When T₂ is off (RC is logic
high), +5V_DC is divided to produce +1.8V_DC at the V_RING
input. When T₂ is on (RC is logic low), its emitter base volt-
age of +0.9V_DC is divided resulting in +0.2V at the anode of
D₆, hence reverse biasing the diode (D₆) and floating the
V_RING pin.
MTU Voltage Application Circuit Overview
According to Bellcore specification TR-NWT-000057, an
MTU voltage may be required by some operating
companies. The minimum allowable voltage to meet MTU
requirements is -42.75V, which is used by measurement
equipment to verify an active line. Also, some facsimile and
answering machines use the MTU voltage as an indication
that the telephone is on-hook or not answered. In addition to
the Bellcore specification, FCC Part 68.306 requires that the
maximum tip to ground or ring to ground voltage not exceed
-56.5V for hazardous voltage limitations. These two require-
ments have been combined and the resulting range is
defined as the MTU voltage. The HC5517 application circuit
can be programmed to any voltage within this range using
the zener clamping circuit.
MTU Voltage Application Circuit Operation
The circuit used to generate the MTU voltage is shown in
Figure 6.
90K
90K
TIP FEED OUTPUT
-
RF
+
RING FEED
AMPLIFIER
90K
V-----B---2--A----T--
+5V
V_REF 3
R₁₉
R₂₄
C₁₆
D₁₁
RC
T₂
FIGURE 6. RING FEED AMPLIFIER CIRCUIT CONNECTIONS
The ring feed amplifier DC output voltage, V_RDC, is a
function of the internal V_BAT/2 reference and external zener
diode D₁₁. When the magnitude of V_BAT/2 is less than the
zener voltage, the zener is off and the input to the ring feed
amplifier is V_BAT/2. When the magnitude of V_BAT/2 is
greater than the zener voltage, the zener conducts and
clamps the noninverting terminal of the ring amplifier to the
zener voltage.
Internal to the HC5517 are connections to the tip feed amplifier
output and V_BAT/2 reference. The DC voltage at the tip feed
output, V_TDC, is a constant -4V during on-hook standby.
MTU Voltage Design Equations
The following equations are used to predict the DC output of
the ring feed amplifier, V_RDC.
V_{-----B-2---A----T- < VZ}
V_RDC
=
2


V_{-----B-2---A----T-}
+4
(EQ. 31)
-^V----^B-₂---^A----^T- ^{≥ V}_Z
V_RDC = 2(–V_Z + (V_CE – V_BE)) + 4 ^{(EQ. 32)}
Where V_Z is the zener diode voltage of D₁₁ and V_CE and
V_BE are the saturation voltages of T₂. Using Equations 31
and 32, the tip-to-ring open-circuit voltage can be calculated
for any value of zener diode and battery voltage.
-^V----^B-₂---^A----^T- ^{< V}_Z
V_OC
=
V_T
D
C
–
2


-^V----^B-₂---^A----^T-_
–4
(EQ. 33)
V_{-----B----A----T-}
2
≥ V_Z
V_OC = V_TDC – 2(–V_Z + (V_CE – V_BE)) – 4
(EQ. 34)
Figure 7 plots V_OC as a function of battery voltage. The
graph illustrates the clamping function of the zener circuitry.
+50
+40
+30
+20
+10
0
-16
-28
-40
-52
-58
-68
-80
FIGURE 7. V_OC AS A FUNCTION OF BATTERY VOLTAGE
MTU Voltage Logic Control
The same pnp transistor, T₂, that is used to control the
centering voltage is also used to control the MTU voltage.
The application circuit uses T₂ to ground or float the anode
of the zener diode D₁₁. When RC is a logic low (T₂ on) the
anode of D₁₁ is referenced to ground through the collector
base junction of the transistor. Current then flows through
the zener, allowing the ring amplifier input to be clamped.
When RC is a logic high (T₂ off) the anode of D₁₁ floats,
inhibiting the clamping action of the zener.
HC5517 Modes of Operation
The four modes of operation of the HC5517 Ringing SLIC
are ringing, on-hook standby, off-hook active and power
denial. Three control signals select the operating mode of
69