HC55183ECMZ datasheet(11/21 Pages) INTERSIL

11 / 21 page

Forward Active

Overview

The forward active mode (FA, 001) is the primary AC

transmission mode of the device. On hook transmission, DC

loop feed and voice transmission are supported during forward

active. Loop supervision is provided by either the switch hook

detector (E0 = 1) or the ground key detector (E0 = 0). The

device may be operated from either high or low battery for on-

hook transmission and low battery for loop feed.

On-Hook Transmission

The primary purpose of on hook transmission will be to

support caller ID and other advanced signalling features.

The transmission over load level while on hook is 3.5VPEAK.

When operating from the high battery, the DC voltages at Tip

and Ring are MTU compliant. The typical Tip voltage is -4V

and the Ring voltage is a function of the battery voltage for

battery voltages less than -60V as shown in Equation 17.

Loop supervision is provided by the switch hook detector at

the DET output. When DET goes low, the low battery should

be selected for DC loop feed and voice transmission.

Feed Architecture

The design implements a voltage feed current sense

architecture. The device controls the voltage across Tip and

Ring based on the sensing of load current. Resistors are

placed in series with Tip and Ring outputs to provide the

current sensing. The diagram below illustrates the concept.

By monitoring the current at the amplifier output, a negative

feedback mechanism sets the output voltage for a defined

load. The amplifier gains are set by resistor ratios (RA, RB,

RC) providing all the performance benefits of matched

resistors. The internal sense resistor, RCS, is much smaller

than the gain resistors and is typically 20

Ω for this device.

The feedback mechanism, KS, represents the amplifier

configuration providing the negative feedback.

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

filter is set by the external capacitor CDC. The value of the

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

will be near VVBL + 4V. The following diagram shows the DC

feed characteristic.

The point on the y-axis labeled VTR(OC) is the open circuit

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 as the short circuit current ISC.

The term ILIM is the programmed current limit, 1760/RIL. The

line segment IA represents the constant current region of the

loop current limit function.

The maximum loop impedance for a programmed loop

current is defined as RKNEE.

RING

(EQ. 17)

FIGURE 3. VOLTAGE FEED CURRENT SENSE DIAGRAM

VIN

VOUT

RCS

FIGURE 4. DC FEED CHARACTERISTIC

m = (

∆VTR/∆IL) = 10kΩ

ILOOP (mA)

ILIM

VTR(OC)

TR OC

()

–

(EQ. 18)

FIGURE 5. ILOOP vs RLOOP LOAD CHARACTERISTIC

RLOOP (Ω)

RKNEE

ILIM

ISC

2RP

LIM

TR OC

() 2RPILIM

–

10K

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

(EQ. 19)

LIM

TR OC

() RLOOPILIM

–

10K

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

(EQ. 20)

KNEE

TR OC

()

LIM

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

(EQ. 21)

HC55180, HC55181, HC55183, HC55184

HC55183ECMZ Datasheet(PDF) 11 Page - Intersil Corporation

HC55183ECMZ Datasheet(HTML) 11 Page - Intersil Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL