HC5526

FN4151 Rev 8.00

Page 10 of 19

August 2003

For loop resistances that result in a tip to ring voltage less than the

saturation guard voltage the loop current is defined as:

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:

where T = 30ms.

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 0

 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.

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 <1.2k

 (RSG = 21.4k) the

SLIC is operating in the constant current feed region and for

resistances >1.2k

 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.

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 RSG 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 RDC decreases. This results in a corresponding

decrease in the loop current. The RSG pin provides the ability

to increase the saturation guard reference voltage beyond

12.5V. Equation 3 gives the relationship between the RSG

HC5526

VTX

RRX

RDC1

RDC2

CDC

RSN

RDC

IRSN

TIP

RING

-2.5V

IRING

ITIP

A2

ITIP

IRING

RSG

RSG

-5V

LOOP CURRENT

CIRCUIT

SATURATION GUARD

CIRCUIT

A1

I1

I2

R1

+

-

+

-

+

-

+

-

-5V

FIGURE 13. DC LOOP CURRENT

-5V

IL

2.5V

RDC1 RDC2

+

--------------------------------------

1000



=

(EQ. 1)

CDC

T

1

RDC1

---------------

1

RDC2

---------------

+







=

(EQ. 2)

01.2K

-50

-40

-30

-20

-10

0

VBAT = -48V, IL = 23mA, RSG = 21.4k

LOOP RESISTANCE (

)

VTIP

VRING

RESISTIVE FEED

REGION

CONSTANT CURRENT

FEED REGION

SATURATION

GUARD VOLTAGE

SATURATION

GUARD VOLTAGE



FIGURE 14. VTR vs RL

0

10

20

30

0

10

20

30

40

50

LOOP CURRENT (mA)

VBAT = -24V, RSG = 

VBAT = -48V, RSG = 21.4k

SATURATION GUARD

RESISTIVE FEED

REGION

CONSTANT CURRENT

FEED REGION

RRSG = 21.4k

100k



100k



4k



1.5k



2k



700



<400



<1.2k



RL

RL

RRSG = 

VOLTAGE, VTR = 38V

SATURATION GUARD

VOLTAGE, VTR = 13V

FIGURE 15. VTR vs IL and RL