ISL5585ECM datasheet(13/23 Pages) RENESAS

13 / 23 page

ISL5585

FN6026 Rev 6.00

Page 13 of 23

October 21, 2004

Substituting Equation 19 into Equation 17 (VIN =0) and

defining

IM = -V2W/ZL results in Equation 37 for VTX.

Combining Equations 36 and 37 results in Equation 38.

A more useful form of the equation is rewritten in terms of

VTX/V2W. A voltage divider equation is written to convert from

EG to V2W as shown in Equation 39.

Substituting ZL = ZO + 2RP and rearranging Equation 39 in

terms of EG results in Equation 40.

Substituting Equation 40 into Equation 38 results in an

equation for 2-wire to 4-wire gain that’s a function of the

synthesized input impedance of the SLIC and the protection

resistors.

If ZL is set to 600, ZO is programmed with RS to be 498.76

(66.5k

/133.33), and RP is equal to 49.9. This results in a 2-

wire to 4-wire gain of 0.416 or -7.6dB.

When the protection resistors are set to zero, the transmit gain

is -6dB.

Transhybrid Gain

The transhybrid gain is defined as the 4-wire to 4-wire gain

(G44).

Understanding Phase Across the ISL5585

4-Wire to 2-Wire Phase

The phase of a signal through the ISL5585 is dependent upon

whether the source is driving the signal 4-wire to 2-wire or 2-

wire to 4-wire.

Figure 6 illustrates the phase of the input signal across the

ISL5585 when the signal is applied at the -IN pin of the

ISL5585 through the RIN resistor. The Transmit Amplifier (TA)

inverts the signal 180 degrees at the VTX pin. The feedback

around the tip amplifier inverts the signal again on the tip lead.

The input signal will cause AC loop current to flow through the

sense resistors in the direction from

V 1 to V2 and V3 to V4. This results in an inverted signal

(referenced from tip) on the VSA and thus the VFB pin. This

out of phase signal is the signal used by the feedback path to

match the line impedance of the 2-wire side.

2-Wire to 4-Wire Phase

Figure 7 Illustrates the phase of the input signal across the

ISL5585 when the signal is applied across tip and ring. When

you’re driving the 2-wire side with a source the ISL5585 looks

like a predetermined impedance (programmed with resistor

RS). The current flows through the 20

 sense resistors in the

direction V2 to V1 and V4 to V3. This results in a non-inverted

signal (referenced from tip) on the VSA and thus the VFB pin.

This signal is then inverted by the TA amplifier and the signal

appearing on the VTX putput is out of phase with the signal on

tip.

Summary of the Phase Through the ISL5585

4-Wire to 2-Wire (VIN to V2W) is 180° out of phase

2-Wire to 4-Wire (V2w to VTX) is 180° out of phase

4-Wire to 4-Wire (VIN to VTX) is 180° out of phase

VTX

V2W

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

ZL 2RP

–

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

(EQ. 37)

G2-4 =

VTX

---------- =

ZL 2RP

–

2ZL 2RP ZO



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

–

2ZL 2RP ZO



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

–

(EQ. 38)

V2W =

ZO + 2RP

ZL ZO

+ 2RP

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







(EQ. 39)

EG = 2V2W

(EQ. 40)

G2-4 =

VTX

V2W

------------ =

ZL 2RP ZO



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







–

0.416

(EQ. 41)

G44

G42 G



RIN

----------







–

ZL 2RP ZO

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







(EQ. 42)

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