HC55120_06 Datasheet, PDF (16/36 Pages) Intersil Corporation – Low Power Universal SLIC Family

HC55120, HC55121, HC55130, HC55140, HC55142, HC55143, HC55150
Off Hook Overhead Voltage
DC FEED CURVE
The off hook overhead
V_BH
V_SAT
V_OH(off)
2.5V
OFF HOOK
OVER HEAD
voltage V_OH(off) at Load is
also independent of the V_BH
battery voltage and remains
constant over temperature.
The required off hook
overhead voltage is the sum
I_LOOP(min)
of the AC and DC voltage
LOOP CURRENT drops across the internal
sense resistors (R_S), the
protection resistors (R_P), the required (peak) off hook
voltage for speech (V_sp(off)) and the required (peak) off hook
voltage for the pulse metering (V_pm(off)), if applicable.
The off hook overhead voltage is defined in Equation 2 and
calculated using Equation 3.
V_{OH(off) at Load} = V_OH(Rsense) + V_sp(off) + V_pm(off) (EQ. 2)
where:
V_{OH(off) at Load} = Off hook overhead voltage at load
V_OH(R_sense) = Required overhead for the DC voltage drop
across sense resistors (2R_S x Iloop_(max))
V_sp(off) = Required (peak) off hook AC voltage for speech
V_pm(off) = Required (peak) off hook AC voltage for pulse
metering
V_{OH(off) at Load}
=
80
×
I_LOOP(max)
+
V_sp(off)
×
⎛
⎜1
⎝
+
-²---^R-----^P---_Z--+--_L---²----^R----^S--_{⎠⎟⎞}
+
V_pm(off)
×
⎛
⎜1
⎝
+
-²---^R-----^P-_Z----_p^+---_m--²----^R----^S--_{⎠⎟⎞}
+
2.0 V
(EQ. 3)
where:
80 = 2R_s + 2R_INT (reference Figure 17)
Z_pm = Pulse metering load impedance (typically 200Ω).
2.0V = Additional off hook overhead voltage requirement
R_SAT Resistance Calculation
The R_SAT 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 R_SAT value for the off
hook conditions and then verify that the on hook conditions
are also satisfied.
DC FEED CURVE
When considering the Off
V_BH
V_SAT
V_OH(off)
2.5V
R_SAT
hook condition, R_SAT is equal
to V_{OH(off) at Load} divided by
Iloop_(min) (Equation 4).
For the given system
requirements (recommended
I_LOOP(min)
application circuit in back of
LOOP CURRENT data sheet): Iloop (min) =
20mA, Iloop (max) = 30mA,
V_{OH(off) AT LOAD}
R_{SAT Vsp(off) = 3.2VPEAK,}
I_LOOP(min)
V_spm(off) = 0V_PEAK,
V_{OH(off) at Load} = 8.34V the
value of R_SAT(off) is equal to 417Ω as calculated in Equation 4.
R_SAT(off)
=
V_{-----O----H----(-o---f--f-)---a---t---L---o---a---d-}
I_LOOP(min)
=
-⁸---^.--³---⁴----^V--
20 m A
=
417 Ω
(EQ. 4)
Before using this R_SAT value, to calculate the RDC_RAC
resistor, we need to verify that the on hook requirements will
also be met.
DC FEED CURVE
V_BH
V_SAT
2.5V
V_OH(on)
R_SAT
The on hook overhead voltage
calculated with the off hook
R_SAT (R_SAT(off)), is given in
Equation 5 and equals 3.0V.
The on hook overhead
calculated with Equation 1
equals 2.85V for the given
ISH-(min)
system requirements
LOOP CURRENT _{(recommended application}
V_{OH(on) AT LOAD}
R_SAT ^{circuit in back of data sheet):}
Switch Hook Detect threshold
ISH-_(min)
R_SAT(on)
=
--²---^.--⁸---⁵----^V---
7.2 m A
=
395 Ω
= 12mA, ISH- = (0.6)12mA =
7.2mA, V_sp(on) = 0.775V_RMS
Thus, the on hook overhead
requirements of 2.85V will be
met if we use the R_SAT(off) value.
V_OH(on) = (ISH-)(R_SAT(off))
(EQ. 5)
V_OH(on) = 7.2mA × 417Ω
V_OH(on) = 3.0V
If the on hook overhead requirement is not met, then we
need to use the R_SAT(on) value to determine the RDC_RAC
resistor value. The external saturation guard resistor
RDC_RAC is equal to 50 times R_SAT.
In the example above R_SAT would equal 417Ω and
RDC_RAC would then equal to 20.85kΩ (closest standard
value is 21kΩ).
RDC_RAC = 50 x R_SAT
(EQ. 6)
The Switch Hook Detect threshold current is set by resistor
R_D and is calculated using Equation 7. For the above
16
FN4659.13
June 1, 2006