HC5523IP datasheet(11/18 Pages) INTERSIL

11 / 18 page

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

TIP

RING

–

600

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

300

----------

(EQ. 24)

465

HOOK to OFF

–

HOOK

–

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

(EQ. 25)

375

OFF

HOOK to ON

–

HOOK

–

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

(EQ. 26)

--------

(EQ. 27)

HC5523

DET

gm1

gm2

gm1(IMETALLIC)

VREF

CURRENT

LOOP

VEE

-5V

GROUND

KEY

COMPARATOR

gm2(ITIP - IRING)

IRD

IGK

RING

TIP

ORDERING INFORMATION

CASE 1

CASE 2

CASE 3

IMETALLIC

←

ILONGITUDINAL

←

ILONGITUDINAL

→

IMETALLIC

→

ILONGITUDINAL

←

ILONGITUDINAL

→

COMPARATOR

DIGITAL MULTIPLEXER

1.25V

FIGURE 18. LOOP CURRENT AND GROUND KEY DETECTORS

HC5523

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