68

Figure 19 illustrates battery backed unbalanced ring injected

ringing. For tip injected ringing just reverse the leads to the

phone. The ringing source could also be balanced.

NOTE: The DET output will toggle at 20Hz because the DT input is

and determine if the DET pin is low for more that half the time, if so

the off-hook condition is indicated.

Longitudinal Impedance

The feedback loop described in Figure 20(A, B) realizes the

desired longitudinal impedances from tip to ground and from

ring to ground. Nominal longitudinal impedance is resistive

and in the order of 22

Ω.

In the presence of longitudinal currents this circuit

attenuates the voltages that would otherwise appear at the

tip and ring terminals, to levels well within the common mode

range of the SLIC. In fact, longitudinal currents may exceed

the programmed DC loop current without disturbing the

SLIC’s VF transmission capabilities.

The function of this circuit is to maintain the tip and ring

longitudinal currents. The differential transconductance

When a longitudinal current is injected onto the tip and ring

appears between the input terminals of the differential

The circuit shown in Figure 20(B) illustrates the tip side of

the longitudinal network. The advantages of a differential

input current source are: improved noise since the noise due

to differential current gain and minimized offset error at the

Operational Amplifier inputs via the two 5k

Ω resistors.

Digital Logic Inputs

Table 1 is the logic truth table for the TTL compatible logic

input pins. The HC5526 has two enable inputs pins (E0, E1)

and two control inputs pins (C1, C2).

The enable pin E0 is used to enable or disable the DET

output pin. The DET pin is enabled if E0 is at a logic level 0

and disabled if E0 is at a logic level 1.

The enable pin E1 gates the ground key detector to the DET

output with a logic level 0, and gates the loop or ring trip

detector to the DET output with a logic level 1.

A combination of the control pins C1 and C2 is used to

select 1 of the 4 possible operating states. A description of

each operating state and the control logic follow:

Open Circuit State (C1 = 0, C2 = 0)

In this state the SLIC is effectively off. All detectors and both

the tip and ring line drive amplifiers are powered down,

presenting a high impedance to the line. Power dissipation is

at a minimum.

HC5526

DET

-

CURRENT

LOOP

-5V

GROUND

KEY

COMPARATOR

RING

TIP

+

-

+

-

CASE 1

CASE 2

CASE 3

←

←

→

COMPARATOR

DIGITAL MULTIPLEXER

+

-

1.25V

+

-

FIGURE 18. LOOP CURRENT AND GROUND KEY DETECTORS

TIP

RING

HC5526

DR

DT

RING TRIP

COMPARATOR

DET

RINGRLY

RING

RELAY

+

-

FIGURE 19. RING TRIP CIRCUIT FOR BATTERY BACKED RINGING

HC5526