8

FN4539.3

DC Loop Feed

The feedback mechanism for monitoring the DC portion of

the loop current is the loop detector. A low pass filter is used

in the feedback to block voice band signals from interfering

with the loop current limit function. The pole of the low pass

external capacitor should be 4.7

µF.

Most applications will operate the device from low battery

while off hook. The DC feed characteristic of the device will

drive Tip and Ring towards half battery to regulate the DC

loop current. For light loads, Tip will be near -4V and Ring

feed characteristic.

Tip to Ring voltage and is defined by the feed battery

voltage.

The curve of Figure 5 determines the actual loop current for

a given set of loop conditions. The loop conditions are

determined by the low battery voltage and the DC loop

impedance. The DC loop impedance is the sum of the

protection resistance, copper resistance (ohms/foot) and the

telephone off hook DC resistance.

The slope of the feed characteristic and the battery voltage

define the maximum loop current on the shortest possible

loop current limit function.

The maximum loop impedance for a programmed loop

constant current feed to constant voltage, resistive feed. The

load characteristic.

Voice Transmission

The feedback mechanism for monitoring the AC portion of

the loop current consists of two amplifiers, the sense

amplifier (SA) and the transmit amplifier (TA). The AC

feedback signal is used for impedance synthesis. A detailed

model of the AC feed back loop is provided below.

blocks the DC component of the loop current. The ground

symbols in the model represent AC grounds, not actual DC

potentials.

Ring voltage and load is calculated using Equation 23.

The transmit amplifier provides the programmable gain

required for impedance synthesis. In addition, the output of

this amplifier interfaces to the CODEC transmit input. The

output voltage is calculated using Equation 24.

Once the impedance matching components have been

selected using the design equations, the above equations

provide additional insight as to the expected AC node

voltages for a specific Tip and Ring load.

FIGURE 4. DC FEED CHARACTERISTIC

m = (

()

–

=

(EQ. 18)

–

10e3

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

+

=

(EQ. 19)

–

10e3

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

+

=

(EQ. 20)

()

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

=

(EQ. 21)

()

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

=

(EQ. 22)

FIGURE 6. AC SIGNAL TRANSMISSION MODEL

TIP

RING

+

-

-IN

VFB

VRX

VTX

R

R

R

R

+

-

+

-

1:1

20

20

0.75R

3R

3R

3R

3R

R/2

8K

–

()

–

10

------

=

(EQ. 23)

8e3

----------





–

=

(EQ. 24)

HC5549