HC55180 datasheet(19/21 Pages) INTERSIL

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Additional Application Diagrams

Reducing Overhead Voltages

The transmission overhead voltage of the device is

internally set to 4V per side. The overhead voltage may be

reduced by injecting a negative DC voltage on the receive

input using a voltage divider (Figure 18). Accordingly, the

2-wire port overload level will decrease the same amount

as the injected offset.

The divider shunt resistance is the parallel combination of

the internal 160k

Ω resistor and the external R

2. The sum of

R1 and R2 should be greater than 500kΩ to minimize the

additional power dissipation of the divider. The DC gain

relationship from the divider voltage, VD, to the Tip and Ring

outputs is shown below.

With a low battery voltage -24V and a divider voltage of

-0.5V, the Tip to Ring voltage is 17V. As a result, the

overhead voltage is reduced from 8V to 7V and the overload

level will decrease from 3.5VPEAK to 3.0VPEAK.

CODEC Ringing Generation

Maximum ringing amplitudes of the device are achieved with

signal levels approximately 2.4VP-P. Therefore the low pass

receive output of the CODEC may serve as the low level ring

generator. The ringing input impedance of 480k

Ω minimum

should not interfere with CODEC drive capability. A single

external capacitor is required to AC coupled the ringing

signal from the CODEC. The circuit diagram for CODEC

ringing is shown below.

Implementing Teletax Signalling

A resistor, RT, is required at the -IN input of the device for

injecting the teletax signal (Figure 19). For most

applications the synthesized device impedance (i.e., 600

Ω)

will not match the 200

Ω teletax impedance. The gain set by

RT cancels the impedance matching feedback with respect

to the teletax injection point. Therefore the device appears

as a low impedance source for teletax. The resistor RT is

calculated using the following equation.

The signal level across a 200

Ω load will be twice the injected

teletax signal level. As the teletax level at VTX will equal the

injection level, set RC = RB for cancellation. The value of RB

is based on the voice band transhybrid balance

requirements. The connection of the teletax source to the

transhybrid amplifier should be AC coupled to allow proper

biasing of the transhybrid amplifier input

Ringing With DC Offsets

The balanced ringing waveform consists of zero DC offset

between the Tip and Ring terminals. However, the linear

amplifier architecture provides control of the DC offset during

ringing. The DC gain is the same as the AC gain, 40V/V per

amplifier. Positive DC offsets applied directly to the ringing

input will shift both Tip and Ring away from half battery

towards ground and battery respectively. A voltage divider

on the ringing input may be used to generate the offset

(Figure 21). The reference voltage, VREF, can be either the

CODEC 2.4V reference voltage or the 5V supply.

An offset during ringing of 30V, would require a DC shift of

15V at Tip and 15V at Ring. The DC offset would be created

by a +0.375V (VD) at the VRS input. The divider resistors

should be selected to minimize the value of the AC coupling

capacitor CRS and the loading of the ring generator and

voltage reference. The ringing input impedance should also

be accounted for in divider resistor calculations.

FIGURE 18. EXTERNAL OVERHEAD CONTROL

HC5518X

VBL

FROM

CODEC

CRX

160k

Ω

1:1

VRX

–

()

–

(EQ. 38)

FIGURE 19. CODEC RINGING INTERFACE

HC5518X

RX OUT

CODEC

160k

Ω

1:1

VRS

480K

VRX

200

S 400

⁄

()

------------------------------------------------------------------- R

(EQ. 39)

FIGURE 20. TELETAX SIGNALLING

CODEC

+2.4V

TX IN

-IN

VFB

VTX

CFB

TELETAX

SOURCE

FIGURE 21. EXTERNAL OVERHEAD CONTROL

VREF

FROM

RING GEN.

CRS

HC5518X

VRS

480K

HC55180, HC55181, HC55183, HC55184

HC55180 Datasheet(PDF) 19 Page - Intersil Corporation

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