HC55151 datasheet(16/35 Pages) INTERSIL

16 / 35 page

4-16

The ROH resistor, which

is used to set the offhook

overhead voltage, is

calculated using

Equations 9 and 10.

IOH is deﬁned as the

difference between the

ILOOP(min) and ISH-.

Substituting Equation 8

for ISH- into Equation 9 and solving for ROH deﬁnes ROH in

terms of ILOOP(min) and RD.

Equation 10 can be used to determine the actual ISH- value

resulting from the RD resistor selected. The value of RD

should be the next standard value that is lower than that

calculated. This will insure meeting the ILOOP(min)

requirement. ROH for the above example equals 39.1k

Ω.

The current limit is set by a single resistor and is calculated

using Equation 11.

The maximum loop

resistance is calculated

using Equation 12. The

resistance of the

protection resistors

(2RP) is subtracted out

to obtain the maximum

loop length to meet the

required off hook

overhead voltage. If RLOOP(MAX) meets the loop length

requirements you are done. If the loop length needs to be

longer, then consider adjusting one of the following: 1) the

SHD threshold, 2) minimum loop current requirement or 3)

the on and off hook signal levels.

SLIC in the Active Mode

Figure 17 shows a simpliﬁed AC transmission model. Circuit

analysis yields the following design equations:

Substitute Equation 14 into Equation 15

Substitute Equation 16 into Equation 17

Substitute Equation 18 into Equation 19

Substituting -VTR/ZL into Equation 20 for IM and rearranging

to solve for VTR results in Equation 21

where:

VRX = The input voltage at the VRX pin.

VA = An internal node voltage that is a function of the loop

current detector and the impedance matching networks.

IX = Internal current in the SLIC that is the difference

between the input receive current and the feedback current.

IM = The AC metallic current.

RP = A protection resistor (typical 30Ω).

ZT = An external resistor/network for matching the line

impedance.

VTX´= The tip to ring voltage at the output pins of the SLIC.

VTR = The tip to ring voltage including the voltage across the

protection resistors.

ZL = The line impedance.

ZTR = The input impedance of the SLIC including the

protection resistors.

(AC) 4-Wire to 2-Wire Gain

The 4-wire to 2-wire gain is equal to VTR/VRX.

From Equation 21 and the relationship ZT = 200(ZTR-2RP).

Notice that the phase of the 4-wire to 2-wire signal is 180o

out of phase with the input signal.

VBH

VSAT

VOH(off)

2.5V

OFF HOOK

LOOP CURRENT

ILOOP(min)

DC FEED CURVE

ISH-

IOH

OVER HEAD

500

----------

500

LOOP(min)- ISH-

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

(EQ. 9)

OH =

D500

DILOOP(min)

- 500(.6)

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

(EQ. 10)

LIM =

1000

LOOP(max)

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

(EQ. 11)

VBH

VSAT

VOH(off)

2.5V

LOOP CURRENT

ILOOP(min)

DC FEED CURVE

RLOOP(MAX)

LOOP(max) =

SAT

OH off

()

[]

–

LOOP(min)

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

-2R

(EQ. 12)

A = IM

80k

----------

200

–

()

(EQ. 13)

-------

–

()

(EQ. 14)

500k

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

500k

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

Node Equation

(EQ. 15)

500k

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

M ZTR

–

()

1000k

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

(EQ. 16)

X500k - VTX′ +IX500k = 0

Loop Equation

(EQ. 17)

TX′

M ZTR

–

()

–

(EQ. 18)

TR-IM2RP +VTX′ = 0

Loop Equation

(EQ. 19)

MZTR

–

(EQ. 20)

TR 1

-----------







– V

(EQ. 21)

4-2 =

-----------

=-2

L +ZTR

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

200

----------





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

–

(EQ. 22)

HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151

HC55151 Datasheet(PDF) 16 Page - Intersil Corporation

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