CPC5712

8

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R02

3.1 Line Interface

Between the CPC5712 and the TIP/RING line is a

high impedance resistive divider network that provides

sufficient impedance to meet the barrier insulation

specifications in safety regulations and comply with

the on-hook DC leakage to ground requirements from

the various network compatibility specifications.

To ensure regulatory compliance, a 20M

 or greater

resistance is required from the individual TIP and

RING leads to the IN+ and IN- inputs. For most

applications where the tip and ring interface does not

have a ground referenced surge protector, Clare

recommends using two 1206-size 10M

 resistors in

series to provide the minimum impedance and to meet

surge requirements. Resistors having a smaller

physical footprint may be used when ground

referenced surge protection is available.

In practice, each 1206-size resistor is capable of

withstanding the 2000V peak waveforms typical of

lightning surges on the phone line. Hence, two 1206

resistors can withstand 4000V lightning pulses.

3.2 Differential Input Resistor

The differential input resistor placed across the IN+

and IN- inputs provides two functions.

From the application perspective, this component

provides a scaled down representation of the tip and

ring line voltage to the CPC5712 inputs. The voltage

applied to the inputs is easily calculated because it is

derived from a simple resistive divider comprising the

tip and ring input resistors and the differential input

resistor.

For improved performance, the CPC5712 signal path

is trimmed at the factory to reduce comparator

detection errors caused by offset currents and

voltages. The CPC5712’s input offset effects are

reduced by trimming the device with an 806k

input

resistor. Using any other value resistor at the inputs

negates the trim and introduces offset errors.

3.3 Voltage Detector Design

From the application requirements given above, the

desired LIU detector threshold voltages are therefore:

and the detector thresholds for the LOOP detector are:

3.3.1 Calculate Resistor Values

From the design equations provided in

Section 2.5 “Detector Threshold Operation” on

page 6 this gives:

Summing these equations provides the following

result:

R1+R2+R3+R4+R5 = 10.12556 R1

and since this sum is bound by:

20k

 < (R1 + R2 + R3 + R4 + R5) < 1M

this reduces to: 20k

 < (10.12556 R1) < 1M

Taking into account the additional constraint of resistor

tolerance, 1% in this example, the range of allowable

values for R1 is further reduced and becomes:

1.995k

 < R1 < 97.782k permitting a value

for R1 to be chosen.

Selecting a standard value from the E96, 1% table for

R1 of 26.7k

 the calculated values for the remaining

resistors becomes:









Since the calculated values of R3 and R5 are not

standard values, a reasonable compromise for these

resistors is: R3=61.9k

, R5=137k. See Figure 1.