HC5513 datasheet(12/20 Pages) RENESAS

12 / 20 page

HC5513

FN3963 Rev.12.00

Page 12 of 20

August 2003

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.

IRD

ITIP IRING

–

600

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

300

----------

(EQ. 24)

465

ION HOOK to OFF

–

HOOK

–

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

(EQ. 25)

375

IOFF HOOK to ON

–

HOOK

–

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

(EQ. 26)

--------

(EQ. 27)

HC5513

DET

gm1

gm2

gm1(IMETALLIC)

VREF

CURRENT

LOOP

VEE

-5V

GROUND

KEY

COMPARATOR

gm2(ITIP - IRING)

IRD

IGK

RING

TIP

CASE 1

CASE 2

CASE 3

IMETALLIC

ILONGITUDINAL

IMETALLIC

ILONGITUDINAL

COMPARATOR

DIGITAL MULTIPLEXER

1.25V

FIGURE 18. LOOP CURRENT AND GROUND KEY DETECTORS

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