HC5513_03 Datasheet, PDF (8/20 Pages) Intersil Corporation – TR909 DLC/FLC SLIC with Low Power Standby

HC5513
Circuit Operation and Design Information
The HC5513 is a current feed voltage sense Subscriber Line
Interface Circuit (SLIC). This means that for short loop
applications the SLIC provides a programed constant current to
the tip and ring terminals while sensing the tip to ring voltage.
The following discussion separates the SLIC’s operation into
its DC and AC path, then follows up with additional circuit
and design information.
Constant Loop Current (DC) Path
SLIC in the Active Mode
The DC path establishes a constant loop current that flows
out of tip and into the ring terminal. The loop current is
programmed by resistors R_DC1, R_DC2 and the voltage on
the R_DC pin (Figure 13). The R_DC voltage is determined by
the voltage across R₁ in the saturation guard circuit. Under
constant current feed conditions, the voltage drop across R₁
sets the R_DC voltage to -2.5V. This occurs when current
flows through R₁ into the current source I₂. The R_DC voltage
establishes a current (I_RSN) that is equal to V_RDC/(R_DC1
+R_DC2). This current is then multiplied by 1000, in the loop
current circuit, to become the tip and ring loop currents.
For the purpose of the following discussion, the saturation
guard voltage is defined as the maximum tip to ring voltage
at which the SLIC can provide a constant current for a given
battery and overhead voltage.
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
×


-------¹--------
R_DC1
+
-_R----_D--¹--_C----₂--_
(EQ. 2)
where T = 30ms.
NOTE: 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)
V_RING
∞
FIGURE 14. V_TR vs R_L
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
8