HC5526

FN4151 Rev 8.00

Page 13 of 19

August 2003

Supervisory Functions

The loop current, ground key and the ring trip detector outputs

are multiplexed to a single logic output pin called DET. See

Table 1 to determine the active detector for a given logic input.

For further discussion of the logic circuitry see section titled

“Digital Logic Inputs”.

Before proceeding with an explanation of the loop current

detector, ground key detector and later the longitudinal

impedance, it is important to understand the difference between a

“metallic” and “longitudinal” loop currents. Figure 18 illustrates 3

different types of loop current encountered.

Case 1 illustrates the metallic loop current. The definition of a

metallic loop current is when equal currents flow out of tip and

into ring. Loop current is a metallic current.

Cases 2 and 3 illustrate the longitudinal loop current. The

definition of a longitudinal loop current is a common mode

current, that flows either out of or into tip and ring

simultaneously. Longitudinal currents in the on-hook state

result in equal currents flowing through the sense resistors R1

and R2 (Figure 18). And longitudinal currents in the off-hook

state result in unequal currents flowing through the sense

resistors R1 and R2. Notice that for case 2, longitudinal

currents flowing away from the SLIC, the current through R1 is

the metallic loop current plus the longitudinal current; whereas

the current through R2 is the metallic loop current minus the

longitudinal current. Longitudinal currents are generated when

the phone line is influenced by magnetic fields (e.g., power

lines).

Loop Current Detector

Figure 18 shows a simplified schematic of the loop current and

ground key detectors. The loop current detector works by

sensing the metallic current flowing through resistors R1 and R2.

This results in a current (IRD) out of the transconductance

amplifier (gm1) that is equal to the product of gm1 and the

metallic loop current. IRD then flows out the RD pin and through

resistor RD to VEE. The value of IRD is equal to:

The IRD current results in a voltage drop across RD that is

compared to an internal 1.25V reference voltage. When the

voltage drop across RD exceeds 1.25V, and the logic is

configured for loop current detection, the DET pin goes low.

The hysteresis resistor RH adds an additional voltage

effectively across RD, causing the on-hook to off-hook

threshold to be slightly higher than the off-hook to on-hook

threshold.

Taking into account the hysteresis voltage, the typical value of

RD for the on-hook to off-hook condition is:

Taking into account the hysteresis voltage, the typical value of

RD for the off-hook to on-hook condition is:

A filter capacitor (CD) in parallel with RD will improve the

accuracy of the trip point in a noisy environment. The value of

this capacitor is calculated using the following Equation:

where: T = 0.5ms.

Ground Key Detector

A simplified schematic of the ground key detector is shown in

Figure 18. Ground key, is the process in which the ring terminal

is shorted to ground for the purpose of signaling an Operator or

seizing a phone line (between the Central Office and a Private

Branch Exchange). The Ground Key detector is activated when

unequal current flow through resistors R1 and R2. This results in

a current (IGK) out of the transconductance amplifier (gm2) that

is equal to the product of gm2 and the differential (ITIP -IRING)

loop current. If IGK is less than the internal current source (I1),

then diode D1 is on and the output of the ground key comparator

is low. If IGK is greater than the internal current source (I1), then

diode D2 is on and the output of the ground key comparator is

high. With the output of the ground key comparator high, and the

logic configured for ground key detect, the DET pin goes low.

The ground key detector has a built in hysteresis of typically

5mA between its trigger and reset values.

Ring Trip Detector

Ring trip detection is accomplished with the internal ring trip

comparator and the external circuitry shown in Figure 19. The

process of ring trip is initiated when the logic input pins are in the

following states: E0 = 0, E1 = 1/0, C1 = 1 and C2 = 0. This logic

condition connects the ring trip comparator to the DET output, and

causes the Ringrly pin to energize the ring relay. The ring relay

connects the tip and ring of the phone to the external circuitry in

Figure 19. When the phone is on-hook the DT pin is more positive

than the DR pin and the DET output is high. For off-hook

conditions DR is more positive than DT and DET goes low. When

DET goes low, indicating that the phone has gone off-hook, the

SLIC is commanded by the logic inputs to go into the active state.

In the active state, tip and ring are once again connected to the

phone and normal operation ensues.

HC5526

VTX

RSN

RTX

RFB

CODEC/

FILTER

I1

I2

VTX

ZRX

ZT

+

-

ZB

VRX

+

-

+

-

FIGURE 17. TRANSHYBRID CIRCUIT

IRD

ITIP IRING

–

600

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

IL

300

----------

==

(EQ. 24)

RD

465

ION HOOK to OFF

–

HOOK

–

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

=

(EQ. 25)

RD

375

IOFF HOOK to ON

–

HOOK

–

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

=

(EQ. 26)

CD

T

RD

--------

=

(EQ. 27)