HC55130IMZ datasheet(16/36 Pages) RENESAS

16 / 36 page

HC55120, HC55121, HC55130, HC55140, HC55142, HC55143, HC55150

FN4659 Rev 13.00

Page 16 of 36

June 1, 2006

Off Hook Overhead Voltage

The off hook overhead

voltage VOH(off) at Load is

also independent of the VBH

battery voltage and remains

constant over temperature.

The required off hook

overhead voltage is the sum

of the AC and DC voltage

drops across the internal

sense resistors (RS), the

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 200).

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 417 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 417 and

RDC_RAC would then equal to 20.85k

 (closest standard

value is 21k

).

The Switch Hook Detect threshold current is set by resistor

RD and is calculated using Equation 7. For the above

VBH

VSAT

VOH(off)

2.5V

OFF HOOK

LOOP CURRENT

ILOOP(min)

DC FEED CURVE

OVER HEAD

(EQ. 2)

VOH off

 at Load

VOH Rsense



Vsp off



Vpm off



(EQ. 3)

VOH(off) at Load

80 ILOOP max





Vsp off



2RP 2RS

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









+ Vpm off



2RP 2RS

Zpm

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









2.0V

VBH

VSAT

VOH(off)

2.5V

LOOP CURRENT

ILOOP(min)

VOH(off) AT LOAD

ILOOP(min)

RSAT

DC FEED CURVE

RSAT(off) =

VOH(off) at Load

ILOOP(min)

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

8.34V

20mA

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

417



(EQ. 4)

VOH(on) AT LOAD

ISH-(min)

RSAT

VBH

VSAT

VOH(on)

2.5V

ISH-(min)

LOOP CURRENT

DC FEED CURVE

RSAT on



2.85V

7.2mA

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

395



RSAT

VOH on



ISH-

 R

SAT off



(EQ. 5)

VOH on



7.2mA 417





VOH on



3.0V

RDC_RAC = 50 x RSAT

(EQ. 6)

HC55130IMZ Datasheet(PDF) 16 Page - Renesas Technology Corp

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