HC5526

FN4151 Rev 8.00

Page 11 of 19

August 2003

resistor value and the programmable saturation guard

reference voltage:

where:

VSGREF = Saturation Guard reference voltage, and,

RSG = Saturation Guard programming resistor.

When the Saturation guard reference voltage is exceeded, the

tip to ring voltage is calculated using Equation 4:

where:

VTR = Voltage differential between tip and ring, and,

RL = Loop resistance.

For on-hook transmission RL = , Equation 4 reduces to:

The value of RSG should be calculated to allow maximum loop

length operation. This requires that the saturation guard

reference voltage be set as high as possible without clipping

the incoming or outgoing VF signal. A voltage margin of -4V on

tip and -4V on ring, for a total of -8V margin, is recommended

as a general guideline. The value of RSG is calculated using

Equation 6:

where:

VBAT = Battery voltage, and

VMARGIN = Recommended value of -8V to allow a maximum

overload level of 3.1VPEAK.

For on-hook transmission RL = , Equation 6 reduces to:

SLIC in the Standby Mode

Overall system power is saved by configuring the SLIC in the

standby state when not in use. In the standby state the tip and

ring amplifiers are disabled and internal resistors are

connected between tip to ground and ring to VBAT. This

connection enables a loop current to flow when the phone

goes off-hook. The loop current detector then detects this

current and the SLIC is configured in the active mode for voice

transmission. The loop current in standby state is calculated as

follows:

where:

IL = Loop current in the standby state,

RL = Loop resistance, and

VBAT = Battery voltage.

(AC) Transmission Path

SLIC in the Active Mode

Figure 16 shows a simplified AC transmission model. Circuit

analysis yields the following design equations:

where:

VTR = Is the AC metallic voltage between tip and ring,

including the voltage drop across the fuse resistors RF,

VTX = Is the AC metallic voltage. Either at the ground

referenced 4-wire side or the SLIC tip and ring terminals,

IM = Is the AC metallic current,

RF = Is a fuse resistor,

ZT = Is used to set the SLIC’s 2-wire impedance,

VRX = Is the analog ground referenced receive signal,

ZRX = Is used to set the 4-wire to 2-wire gain,

EG = Is the AC open circuit voltage, and

ZL = Is the line impedance.

(AC) 2-Wire Impedance

The AC 2-wire impedance (ZTR) is the impedance looking into

the SLIC, including the fuse resistors, and is calculated as

follows:

Let VRX = 0. Then from Equation 10:

ZTR is defined as:

Substituting in Equation 9 for VTR:

Substituting in Equation 12 for VTX:

VSGREF

12.5 510



5

RSG

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

+

=

(EQ. 3)

VTR

RL

16.66 5 10

5



RSG



+

RL RDC1 RDC2

+

 600



+

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



=

(EQ. 4)

VTR

16.66 510

5



RSG

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

+

=

(EQ. 5)

RSG

510

5



VBAT VMARGIN

–



1

RDC1 RDC2

+



600RL

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

+







16.66V

–



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

=

(EQ. 6)

RSG

510

5



VBAT VMARGIN 16.66V

–

–

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

=

(EQ. 7)

IL

VBAT 3V

–

RL 1800

+

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



(EQ. 8)

VTR

VTX IM 2RF



+

=

(EQ. 9)

VTX

ZT

-----------

VRX

ZRX

-----------

+

IM

1000

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

=

(EQ. 10)

VTR

EG IM ZL



–

=

(EQ. 11)

VTX

ZT

IM

1000

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



=

(EQ. 12)

ZTR

VTR

IM

-----------

=

(EQ. 13)

ZTR

VTX

IM

-----------

2RF IM



IM

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

+

=

(EQ. 14)

ZTR

ZT

1000

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

2RF

+

=

(EQ. 15)