72

Operation of the Battery Switch

The battery switch is used to select between the off-hook

battery of -24V and the ringing/standby battery of -80V.

is off and the -24V battery is supplied to the SLIC through

The purpose of the transhybrid circuit is to remove the

receive signal (V-REC) from the transmit signal (V-XMIT),

thereby preventing an echo on the transmit side. This is

accomplished by using an external op amp (usually part of

the CODEC) and by the inversion of the signal from the

transmit port (OUT1).

The external transhybrid circuit is shown in Figure 10. The

fore omitted from the analysis. The input signal (V-REC) will

are equal and opposite in phase. A node analysis yields the

following equation:

Given that OUT1 is equal to -1/3 of V-REC (Equation 16) and

be achieved by using 1% resistors values.

Transhybrid balance of the pulse metering signal is

accomplished in 2 stages. The first stage uses the SLIC’s

internal op amp to invert the phase of the pulse metering sig-

nal. The second stage sums the inverted pulse metering sig-

nal with the incoming signal for cancellation in the

transhybrid amplifier. A third network can be added to offset

both tip and ring by the peak amplitude of the pulse metering

signal. This will allow both the maximum voice and pulse

metering signals to occur at the same time with no distortion.

Pulse Metering

Pulse metering or Teletax is used outside the United States

for billing purposes at pay phones. A 12kHz or 16kHz burst

is injected into the 4-wire side of the SLIC and transmitted

across the tip and ring lines from the central office to the pay

phone. For more information about pulse metering than cov-

ered here reference application note AN9608 “Implementing

Pulse Metering for the HC5509 Series of SLICs”.

TABLE 2. HC5517 APPLICATION CIRCUIT OPERATING MODES SUMMARY

BATTERY

SWITCH

F1

RC

MODE

DETECTORS VALID

APPLICATION CIRCUIT VALID

SHD

RTD

MTU

CENTERING

-24V

0

0

Power Denial

-24V

0

1

Invalid

-24V

1

0

Off-Hook Active

√

-24V

1

1

Invalid

-80V

0

0

Power Denial

√

-80V

0

1

Invalid

-80V

1

0

On-Hook Standby

√√

-80V

1

1

Ringing

(Note)

√√

NOTE: During Ringing, the SHD output will be active for both on-hook and off-hook conditions. The AC current, for the on-hook condition, exceeds

the SHD threshold of 12mA. Valid off-hook detection during ringing is provided by the RTD output only.

VREC

–

R

2

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

OUT1

R

3

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

+

0

=

(EQ. 36)

R

2

R

–

3

V REC

–

OUT1

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

•

=

(EQ. 37)

FIGURE 10. TRANSHYBRID CIRCUIT (VOICE SIGNAL)

EXTERNAL

TRANSHYBRID CIRCUIT

SUMMING NODE CANCELS OUT

INCOMING AC TRANSMISSION FROM

OUT GOING TRANSMISSION

INCOMING

AC TRANSMISSION

180

PHASE

SHIFT

OF AC

SIGNAL

-IN1

OUT1

V-XMIT

V-REC

HC5517

OUTGOING

AC TRANSMISSION

+

-

+

-

HC5517