HC5526 Datasheet, PDF (8/18 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5526
I_{TIP TIP}
RING
I_RING
+
-
I_TIP
I_RING
LOOP CURRENT
CIRCUIT
I_RSN
-
+
HC5526
SATURATION GUARD
CIRCUIT
A₁
I₁
-5V
A₂
I₂
-5V
FIGURE 13. DC LOOP CURRENT
-
+
R_1-
+
V_TX
RSN
R_DC
-2.5V
R_RX
R_DC1
R_DC2
C_DC
R_SG
R_SG
-5V
For loop resistances that result in a tip to ring voltage less than
the saturation guard voltage the loop current is defined as:
I_L
=
-------------²----^.-⁵----^V---------------
R_DC1 + R_DC2
×
1000
(EQ. 1)
where: I_L = Constant loop current.
R_DC1 and R_DC2 = Loop current programming resistors.
Capacitor C_DC between R_DC1 and R_DC2 removes the VF
signals from the battery feed control loop. The value of C_DC
is determined by Equation 2:
C_DC
=
T
×


R^-----_D^--1--_C^-----₁^-
+
R^-----_D^--1--_C^-----₂^{-}
(EQ. 2)
where T = 30ms.
The minimum C_DC value is obtained if R_DC1 = R_DC2.
Figure 14 illustrates the relationship between the tip to ring
voltage and the loop resistance. For a 0Ω loop resistance both
tip and ring are at V_BAT/2. As the loop resistance increases, so
does the voltage differential between tip and ring. When this
differential voltage becomes equal to the saturation guard
voltage, the operation of the SLIC’s loop feed changes from a
constant current feed to a resistive feed. The loop current in the
resistive feed region is no longer constant but varies as a
function of the loop resistance.
V_BAT = -48V, I_L = 23mA, R_SG = 21.4kΩ
0
SATURATION
V_TIP
GUARD VOLTAGE
-10
CONSTANT CURRENT
FEED REGION
-20
RESISTIVE FEED
REGION
-30
-40
-50
0
SATURATION
GUARD VOLTAGE
1.2K
LOOP RESISTANCE (Ω)
FIGURE 14. V_TR vs R_L
V_RING
∞
Figure 15 shows the relationship between the saturation guard
voltage, the loop current and the loop resistance. Notice from
Figure 15 that for a loop resistance <1.2kΩ (R_SG = 21.4kΩ) the
SLIC is operating in the constant current feed region and for
resistances >1.2kΩ the SLIC is operating in the resistive feed
region. Operation in the resistive feed region allows long loop
and off-hook transmission by keeping the tip and ring voltages
off the rails. Operation in this region is transparent to the
customer.
50
V_BAT = -48V, R_SG = 21.4kΩ
40
30
CONSTANT CURRENT
FEED REGION
SATURATION GUARD
VOLTAGE, V_TR = 38V
20
V_BAT = -24V, R_SG = ∞
10 RESISTIVE FEED
REGION
0
0
10
20
30
LOOP CURRENT (mA)
SATURATION GUARD
VOLTAGE, V_TR = 13V
R_L 100kΩ
R_L 100kΩ
4kΩ
1.5kΩ
2kΩ
700Ω
<1.2kΩ R_RSG = 21.4kΩ
<400Ω R_RSG = ∞ Ω
FIGURE 15. V_TR vs I_L and R_L
The Saturation Guard circuit (Figure 13) monitors the tip to
ring voltage via the transconductance amplifier A₁. A₁
generates a current that is proportional to the tip to ring
voltage difference. I₁ is internally set to sink all of A₁’s
current until the tip to ring voltage exceeds 12.5V. When the
tip to ring voltage exceeds 12.5V (with no R_SG resistor) A₁
supplies more current than I₁ can sink. When this happens
A₂ amplifies its input current by a factor of 12 and the current
through R₁ becomes the difference between I₂ and the
output current from A₂. As the current from A₂ increases, the
voltage across R₁ decreases and the output voltage on R_DC
decreases. This results in a corresponding decrease in the
loop current. The R_SG pin provides the ability to increase the
saturation guard reference voltage beyond 12.5V. Equation 3
64