HC55185ECMZ datasheet(13/20 Pages) RENESAS

13 / 20 page

HC55185

FN4831 Rev 15.00

Page 13 of 20

February 9, 2012

this amplifier interfaces to the CODEC transmit input. The

output voltage is calculated using Equation 26.

Once the impedance matching components have been

selected using the design equations, the above equations

provide additional insight as to the expected AC node

voltages for a specific Tip and Ring load.

Transhybrid Balance

The final step in completing the impedance synthesis design

is calculating the necessary gains for transhybrid balance.

The AC feed back loop produces an echo at the VTX output

of the signal injected at VRX. The echo must be cancelled to

maintain voice quality. Most applications will use a summing

amplifier in the CODEC front end as shown below to cancel

the echo signal.

The resistor ratio, RF/RB, provides the final adjustment for

the transmit gain, GTX. The transmit gain is calculated using

Equation 27.

Most applications set RF = RB, hence the device 2-wire to

4-wire equals the transmit gain. Typically RB is greater than

20k

 to prevent loading of the device transmit output.

The resistor ratio, RF/RA, is determined by the transhybrid

gain of the device, G44. RF is previously defined by the

transmit gain requirement and RA is calculated using

Equation 28.

Power Dissipation

The power dissipated by the device during on hook

transmission is strictly a function of the quiescent currents

for each supply voltage during Forward Active operation.

Off hook power dissipation is increased above the quiescent

power dissipation by the DC load. If the loop length is less

than or equal to RKNEE, the device is providing constant

current, IA, and the power dissipation is calculated using

Equation 30.

If the loop length is greater than RKNEE , the device is

operating in the constant voltage, resistive feed region. The

power dissipated in this region is calculated using Equation 31.

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 in Figure 9.

During forward active, the current from source I1 charges

the external timing capacitor CPOL and the switch is open.

VVTX

VSA

8e3

----------





–

(EQ. 26)

FIGURE 8. TRANSHYBRID BALANCE INTERFACE

HC5518x

CODEC

+2.4V

RX OUT

TX IN

-IN

VRX

VTX

1:1

GTX

– 24

--------







(EQ. 27)

G44

----------

(EQ. 28)

PFAQ VBH I



BHQ

VBL IBLQ



VCC ICCQ



(EQ. 29)

PFA IA



PFA Q



VBLxIA



RLOOPxI2A



–

(EQ. 30)

PFA IB



PFA Q



VBLxIB



RLOOPxI2B



–

(EQ. 31)

FIGURE 9. REVERSAL TIMING CONTROL

CPOL

POL

75k



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