HC55183ECMZ datasheet(13/21 Pages) INTERSIL

13 / 21 page

Since the current relationships are different for constant

current versus constant voltage, the region of device

operation is critical to valid power dissipation calculations.

Reverse Active

Overview

The reverse active mode (RA, 011) provides the same

functionality as the forward active mode. On hook

transmission, DC loop feed and voice transmission are

supported. Loop supervision is provided by either the switch

hook detector (E0 = 1) or the ground key detector (E0 = 0).

The device may be operated from either high or low battery.

During reverse active the Tip and Ring DC voltage

characteristics exchange roles. That is, Ring is typically 4V

below ground and Tip is typically 4V more positive than

battery. Otherwise, all feed and voice transmission

characteristics are identical to forward active.

Silent Polarity Reversal

Changing from forward active to reverse active or vice versa

is referred to as polarity reversal. Many applications require

slew rate control of the polarity reversal event. Requirements

range from minimizing cross talk to protocol signalling.

The device uses an external low voltage capacitor, CPOL, to

set the reversal time. Once programmed, the reversal time

will remain nearly constant over various load conditions. In

addition, the reversal timing capacitor is isolated from the AC

loop, therefore loop stability is not impacted.

The internal circuitry used to set the polarity reversal time is

shown below.

During forward active, the current from source I1 charges

the external timing capacitor CPOL and the switch is open.

The internal resistor provides a clamping function for

voltages on the POL node. During reverse active, the switch

closes and I2 (roughly twice I1) pulls current from I1 and the

timing capacitor. The current at the POL node provides the

drive to a differential pair which controls the reversal time of

the Tip and Ring DC voltages.

Where

∆time is the required reversal time. Polarized

capacitors may be used for CPOL. The low voltage at the

POL pin and minimal voltage excursion

±0.75V, are well

suited to polarized capacitors.

Power Dissipation

The power dissipation equations for forward active operation

also apply to the reverse active mode.

Ringing

Overview

The ringing mode (RNG, 100) provides linear amplification to

support a variety of ringing waveforms. A programmable ring

trip function provides loop supervision and auto disconnect

upon ring trip. The device is designed to operate from the

high battery during this mode.

Architecture

The device provides linear amplification to the signal applied

to the ringing input, VRS. The differential ringing gain of the

device is 80V/V. The circuit model for the ringing path is

shown in the following figure.

The voltage gain from the VRS input to the Tip output is

40V/V. The resistor ratio provides a gain of 8 and the current

mirror provides a gain of 5. The voltage gain from the VRS

input to the Ring output is -40V/V. The equations for the Tip

and Ring outputs during ringing are provided below.

When the input signal at VRS is zero, the Tip and Ring

amplifier outputs are centered at half battery. The device

provides auto centering for easy implementation of

sinusoidal ringing waveforms. Both AC and DC control of the

Tip and Ring outputs is available during ringing. This feature

allows for DC offsets as part of the ringing waveform.

Ringing Input

The ringing input, VRS, is a high impedance input. The high

impedance allows the use of low value capacitors for AC

coupling the ring signal. The VRS input is enabled only

during the ringing mode, therefore a free running oscillator

may be connected to VRS at all times.

When operating from a battery of -100V, each amplifier, Tip

and Ring, will swing a maximum of 95VP-P . Hence, the

maximum signal swing at VRS to achieve full scale ringing is

FIGURE 8. REVERSAL TIMING CONTROL

CPOL

POL

75k

Ω

POL

∆time

75000

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

(EQ. 30)

FIGURE 9. LINEAR RINGING MODEL

TIP

RING

VRS

R/8

5:1

800K

VBH

-----------

VRS

()

(EQ. 31)

-----------

VRS

()

–

(EQ. 32)

HC55180, HC55181, HC55183, HC55184

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