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HC5513

SLIC Subscriber Line Interface Circuit

Harris Semiconductor 

HC5513

ITIP TIP

RING

IRING

+

-

ITIP

IRING

-

+

HC5513

LOOP CURRENT

CIRCUIT

IRSN

SATURATION GUARD

CIRCUIT

A1

I1

-5V

A2

I2

-5V

-

+

R1-

+

FIGURE 13. DC LOOP CURRENT

VTX

RSN

RDC

-2.5V

RRX

RDC1

RDC2

CDC

RSG

RSG

-5V

constant current feed conditions, the voltage drop across R1

sets the RDC voltage to -2.5V. This occurs when current

flows through R1 into the current source I2. The RDC voltage

establishes a current (IRSN) that is equal to VRDC/(RDC1

+RDC2). 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:

IL = R-----D----C----2-1---.-+5----V-R----D-----C----2- × 1000

(EQ. 1)

where: IL = Constant loop current.

RDC1 and RDC2 = Loop current programming resistors.

Capacitor CDC between RDC1 and RDC2 removes the VF

signals from the battery feed control loop. The value of CDC

is determined by Equation 2:

CDC = T × -R----D--1--C-----1- + R-----D--1--C-----2-

(EQ. 2)

where T = 30ms

NOTE: The minimum CDC value is obtained if RDC1 = RDC2

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 VBAT/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.

VBAT = -48V, IL = 23mA, RSG = 21.4kΩ

0

SATURATION

VTIP

GUARD VOLTAGE

-10

CONSTANT CURRENT

FEED REGION

-20

RESISTIVE FEED

REGION

-30

-40

-50

0

SATURATION

GUARD VOLTAGE

1.2K

LOOP RESISTANCE (Ω)

VRING

∞

FIGURE 14. VTR vs RL

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 (RSG =

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 trans-

parent to the customer.

50

VBAT = -48V, RSG = 21.4kΩ

40

30

CONSTANT CURRENT

FEED REGION

SATURATION GUARD

VOLTAGE, VTR = 38V

20

VBAT = -24V, RSG = ∞

10 RESISTIVE FEED

REGION

0

0

10

20

30

LOOP CURRENT (mA)

SATURATION GUARD

VOLTAGE, VTR = 13V

RL 100kΩ

RL 100kΩ

4kΩ

1.5kΩ

2kΩ

700Ω

<1.2kΩ RRSG = 21.4kΩ

<400Ω RRSG = ∞ Ω

FIGURE 15. VTR vs IL and RL

8

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