HC55151 datasheet(14/35 Pages) INTERSIL

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4-14

On Hook Overhead Voltage

The on hook overhead voltage

at the load (VOH(on) at Load)

is independent of the VBH

battery voltage. Once set, the

on hook voltage remains

constant as the VBH battery

voltage changes. The on hook

voltage also remains constant

over temperature and line

leakages up to 0.6 times the

Switch Hook Detect threshold (ISHD). The maximum loop

current for a constant on hook overhead voltage is deﬁned

as ISH-.

The on hook overhead voltage, required for a given signal

level at the load, must take into account the AC voltage drop

across the 2 external protection resistors (RP) and the 2

internal sense resistors (RS) as shown in Figure 16. The AC

on hook overload voltage is calculated using Equation 1.

where

VOH(on) at Load = On hook overhead voltage at load

Vsp(on) = Required on hook transmission for speech

RP = Protection Resistors (Typically 30Ω)

RS = Internal Sense Resistors (40Ω)

ZL = AC load impedance for (600Ω)

1.5V = Additional on hook overhead voltage requirement

To account for any process and temperature variations in the

performance of the SLIC, 1.5V is added to the overhead

voltage requirement for the on hook case in Equation 1 and

2.0V for the off hook case in Equation 3. Note the 2.5V

overhead is automatically generated in the SLIC and is not

part of the external overhead programming.

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

†Internal overhead voltage automatically generated by the SLIC.

FIGURE 15. UniSLIC14 DC FEED CURVE

VBH

2.5V

†

VOH(off) AT LOAD

ISH-

ILOOP(min)

ILOOP(max)

60k

Ω SLOPE

RLOOP(max)

SLIC SELF PROGRAMMING

LOOP CURRENT (mA)

IOH

CONSTANT

CURRENT

REGION

RSAT

VOH(on) AT LOAD

ISHD

VBH

VOH(on)

2.5V

ON HOOK

ISH-

LOOP CURRENT

DC FEED CURVE

OVERHEAD

ISH- = ISHD(0.6)

ISHD

(EQ. 1)

OH on

() at Load

sp on

()

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







1.5V

FIGURE 16. OVERHEAD VOLTAGE OF THE TIP AND RING

AMPLIFIERS

TIP AND RING

INTERNAL SENSE

EXTERNAL PROTECTION

RESISTOR

2RP

2RS

AMPLIFIERS

OVERHEAD VOLTAGE

RESISTORS

Where:

REQUIRED

VOH (ON, OFF)

VZL is the required on hook or offhook

transmission delivered to the load.

(LOAD)

UniSLIC14

VZL

OH ON OFF

()

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







VBH

VSAT

VOH(off)

2.5V

OFF HOOK

LOOP CURRENT

ILOOP(min)

DC FEED CURVE

OVER HEAD

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

HC55151 Datasheet(PDF) 14 Page - Intersil Corporation

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