HC55151 datasheet(15/35 Pages) INTERSIL

15 / 35 page

4-15

protection resistors (RP), the required (peak) off hook

voltage for speech (Vsp(off)) and the required (peak) off hook

voltage for the pulse metering (Vpm(off)), if applicable.

The off hook overhead voltage is deﬁned in Equation 2 and

calculated using Equation 3.

where:

VOH(off) at Load = Off hook overhead voltage at load

VOH(Rsense) = Required overhead for the DC voltage drop

across sense resistors (2RS x Iloop(max))

Vsp(off) = Required (peak) off hook AC voltage for speech

Vpm(off) = Required (peak) off hook AC voltage for pulse

metering

where:

80 = 2Rs + 2RINT (reference Figure 17)

Zpm = Pulse metering load impedance (typically 200Ω).

2.0V = Additional off hook overhead voltage requirement

RSAT Resistance Calculation

The RSAT resistance of the DC feed curve is used to

determine the value of the RDC_RAC resistor (Equation 6).

The value of this resistor has an effect on both the on hook

and off hook overheads. In most applications the off hook

condition will dominate the overhead requirements.

Therefore, we’ll start by calculating the RSAT value for the off

hook conditions and then verify that the on hook conditions

are also satisﬁed.

When considering the Off

hook condition, RSAT is equal

to VOH(off) at Load divided by

Iloop(min) (Equation 4).

For the given system

requirements (recommended

application circuit in back of

data sheet): Iloop (min) =

20mA, Iloop (max) = 30mA,

Vsp(off) = 3.2VPEAK,

Vspm(off) = 0VPEAK,

VOH(off) at Load = 8.34V the

value of RSAT(off) is equal to 417Ω as calculated in Equation 4.

Before using this RSAT value, to calculate the RDC_RAC

resistor, we need to verify that the on hook requirements will

also be met.

The on hook overhead voltage

calculated with the off hook

RSAT (RSAT(off)), is given in

Equation 5 and equals 3.0V.

The on hook overhead

calculated with Equation 1

equals 2.85V for the given

system requirements

(recommended application

circuit in back of data sheet):

Switch Hook Detect threshold

= 12mA, ISH- = (0.6)12mA =

7.2mA, Vsp(on) = 0.775VRMS

Thus, the on hook overhead

requirements of 2.85V will be

met if we use the RSAT(off) value.

If the on hook overhead requirement is not met, then we

need to use the RSAT(on) value to determine the RDC_RAC

resistor value. The external saturation guard resistor

RDC_RAC is equal to 50 times RSAT.

In the example above RSAT would equal 417Ω and

RDC_RAC would then equal to 20.85k

Ω (closest standard

value is 21k

Ω).

The Switch Hook Detect threshold current is set by resistor

RD and is calculated using Equation 7. For the above

example RD is calculated to be 41.6kΩ (500/12mA). The

next closest standard value is 41.2k

Ω.

The true value of ISH-, for the selected value of RD is given

by Equation 8:

For the example above, ISH- equals 7.28mA (500 x 0.6/ 41.2K).

Verify that the value of ISH- is above the suspected line

leakage of the application. The UniSLIC family will provide a

constant on hook voltage level for leakage currents up to this

value of line leakage.

(EQ. 2)

OH off

() at Load

OH Rsense

() Vsp off

()

pm off

()

(EQ. 3)

OH(off) at Load

LOOP max

()

sp off

()

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







pm off

()

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







2.0V

VBH

VSAT

VOH(off)

2.5V

LOOP CURRENT

ILOOP(min)

VOH(off) AT LOAD

ILOOP(min)

RSAT

DC FEED CURVE

SAT(off) =

OH(off) at Load

LOOP(min)

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

8.34V

20mA

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

417

Ω

(EQ. 4)

VOH(on) AT LOAD

ISH-(min)

RSAT

VBH

VSAT

VOH(on)

2.5V

ISH-(min)

LOOP CURRENT

DC FEED CURVE

SAT on

()

2.85V

7.2mA

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

395

Ω

RSAT

OH on

()

ISH-

() R

SAT off

()

(EQ. 5)

OH on

()

7.2mA

417

Ω

OH on

()

3.0V

RDC_RAC = 50 x R

SAT

(EQ. 6)

D =

500

SHD

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

(EQ. 7)

ISH- =

500

----------

(0.6)

(EQ. 8)

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

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