HC5523IP datasheet(13/22 Pages) INTERSIL

13 / 22 page

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.

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

not completely filtered by CRT. Software can examine the duty cycle

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

voltages symmetrically around VBAT/2, in the presence of

longitudinal currents. The differential transconductance

amplifiers GT and GR accomplish this by sourcing or sinking

the required current to maintain VC at VBAT/2.

When a longitudinal current is injected onto the tip and ring

inputs, the voltage at VC moves from it’s equilibrium value

VBAT/2. When VC changes by the amount DVC, this change

appears between the input terminals of the differential

transconductance amplifiers GT and GR. The output of GT

and GR are the differential currents ∆I1 and ∆I2, which in turn

feed the differential inputs of current sources IT and IR

respectively. IT and IR have current gains of 250 single

ended and 500 differentially, thus leading to a change in IT

and IR that is equal to 500(∆I) and 500(∆I2).

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 current source 2IO is now correlated, power savings due

to differential current gain and minimized offset error at the

Operational Amplifier inputs via the two 5k

Ω resistors.

TIP

RING

HC5523

RRT

VBAT

ERG

RING TRIP

COMPARATOR

DET

CRT

RINGRLY

RING

RELAY

FIGURE 19. RING TRIP CIRCUIT FOR BATTERY BACKED

RINGING

HC5523

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