HC55151 datasheet(20/35 Pages) INTERSIL

20 / 35 page

4-20

The GKD_LVM output is disabled (TTL high level) during the

ringing state. Reference the Section titled “Ringing the

Phone” for more information.

Forward Active State (C3 = 0, C2 = 1, C1 = 0)

In this state, the SLIC is fully functional. The tip voltage is more

positive than the ring voltage. The tip and ring output voltages

are an unbalanced DC feed, reference Figure 13. Both SHD

and GKD supervisory functions are active. Reference the

section titled “DC Feed Curve” for more information.

Test Active State (C3 = 0, C2 = 1, C1 = 1)

Proper operation of the Test Active State requires the

previous state be the Forward Active state to determine the

on hook or off hook status of the line. In this state, the SLIC

can perform two different tests.

If the subscriber is on hook when the state is entered, a

loopback test is performed by switching an internal 600

Ω

resistor between tip and ring. The current flows through the

internal 600

Ω is unidirectional via blocking diodes. (Cannot be

used in reverse.) When the loopback current flows, the SHD

output will go low and remain there until the state is exited. This

is intended to be a short test since the ability to detect

subscriber off hook is lost during loopback testing. Reference

the section titled “Loopback Tests” for more information.

If the subscriber is off hook when the state is entered, a Line

Voltage Measurement test is performed. The output of the

GKD_LVM pin is a pulse train. The pulse width of the active low

portion of the signal is proportional to the voltage across the tip

and ring pins. If the loop length is such that the SLIC is

operating in constant current, the tip to ring voltage can be used

to determine the length of the line under test. The longer the

line, the larger the tip to ring voltage and the wider the pulse.

This relationship can determine the length of the line for setting

gains in the system. Reference the section titled “Operation of

Line Voltage Measurement” for more information.

Tip Open State (C3 = 1, C2 = 0, C1 = 0)

In this state, the tip output is in a high impedance state

(>250k

Ω) and the ring output is capable of full operation, i.e.

has full longitudinal current capability. The Tip Open/Ground

Start state is used to interface to a PBX incoming 2-wire

trunk line. When a ground is applied through a resistor to the

ring lead, this current is detected and presented as a TTL

logic low on the SHD and GKD_LVM output pins.

Reserved (C3 = 1, C2 = 0, C1 = 1)

This state is undeﬁned and reserved for future use.

Reverse Active State (C3 = 1, C2 = 1, C1 = 0)

In this state, the SLIC is fully functional. The ring voltage is

more positive than the tip voltage. The tip and ring output

voltages are an unbalanced DC feed, reference Figure 13.

The polarity reversal time is determined by the RC time

constant of the RSYNC_REV resistor and the

CRT_REV_LVM capacitor. Capacitor CRT_REV_LVM

performs three different functions: Ring trip filtering, polarity

reversal time and line voltage measurement. It is

recommended that programming of the reversal time be

accomplished by changing the value of RSYNC_REV resistor

(see Figure 18). The value of RSYNC_REV resistor is limited

between 34.8K (10ms) and 73.2k (21ms). Equation 39 gives

the formula for programming the reversal time.

Both SHD and GKD supervisory functions are active.

Reference the section titled “Polarity Reversal” for more

information.

Test Reversal Active State (C3 = 1, C2 = 1, C1 = 1)

Proper operation of the Test Reversal Active State requires

the previous state be the Reverse Active state to determine

the on hook or off hook status of the line.

If the subscriber is on hook when the state is entered, the

SLIC’s tip and ring voltages are the same as the Reverse

Active state. The SHD output will go low when the subscriber

goes off hook and the GKD_LVM output is disabled (TTL

level high). (Note: operation is the same as the Reverse

Active state with the GKD_LVM output disabled.)

If the subscriber is off hook when the state is entered, a

Line Voltage Measurement test is performed.

The output of the GKD_LVM pin is a pulse train. The pulse width

of the active low portion of the signal is proportional to the voltage

across the tip and ring pins. If the loop length is such that the

SLIC is operating in constant current mode, the tip to ring voltage

can be used to determine the length of the line under test. The

longer the line, the larger the tip to ring voltage and the wider the

pulse. This relationship can determine the length of the line for

setting gains in the system. Reference the section titled

“Operation of Line Voltage Measurement” for more information.

Thermal Shutdown

The UniSLIC14’s thermal shutdown protection is invoked if a

fault condition causes the junction temperature of the die to

exceed about 175oC. Once the thermal limit is exceeded,

both detector outputs go low (SHD and GKD_LVM) and one

of two things can happen.

For marginal faults where loop current is ﬂowing during the

time of the over-temperature condition, foldback loop current

limiting reduces the loop current by reducing the tip to ring

voltage. An equilibrium condition will exist that maintains the

junction temperature at about 175oC until the fault condition

is removed.

For short circuit faults (tip or ring to ground, or to a supply,

etc.) that result in an over-temperature condition, the

foldback current limiting will try to maintain an equilibrium at

about 175oC. If the junction temperature keeps rising, the

device will thermally shutdown and disconnect tip and ring

until the junction temperature falls to approximately 150oC.

RSYNC

REV

–

3.47k

Ω ReversalTime ms

()

(EQ. 39)

HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151

HC55151 Datasheet(PDF) 20 Page - Intersil Corporation

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