AN45
Considering the resistance of 26 gauge telephone wire,
which is 0.45 Ω per feet, this equation becomes the
following:
0.6 V + 80(ILIM + IBJTBIAS
)
------------------------------------------------------------------------
IBAT = ILIM + IBJTBIAS
+
5100
Equation 5
where I
is the current limit set by Register 71, and
is the bipolar biasing current set by the direct
LIM
VRINGrms
× 2
7000
NREN
I
BJTBIAS
---------------------------------------
-----------------
+ 2 × Dist × 0.045 + 160
VTR_PK
=
×
7000 ⁄ NREN
Register 65.
Equation 1
There are two power equations for different track
settings. For TRACK = 1, V
is allowed to track the
BAT
line resistance to minimize power consumption. The
power equation for this mode is as follows:
The required V
is equal to V
plus VCMR, which
TR_PK
BAT
is the voltage drop across the linefeed circuit. The
VCMR voltage is set by the indirect Register 40 and
recommended to be 1.5 V for most applications.
POFFHOOK = IBAT(VCM + VOV + ILIM × RLOOPMAX
)
Equation 6
VBAT = VTR_PK + VCMR
where V
is set by the direct Register 73, and V
is
CM
OV
set by Register 66. R
is the maximum total loop
Equation 2
LOOPMAX
resistance (R
+ Phone’s R + R ) where R is the
LINE
DC S S
internal series resistance.
The worst-case peak current for NREN load is when the
load is connected with a short loop of negligible line
resistance.
For TRACK = 0, V can ramp up quickly to support
BAT
the brief on-hook voltage measurement feature, the
power equation is as follows:
VTR_PK
POFFHOOK = IBAT × VBATL
---------------------------------------
=
IPK
(7000 ⁄ NREN)
Equation 7
where V
is set by the direct Register 75.
BATL
This yields an average current equation:
If the off-hook power consumption is greater than the
power during ringing, the dc-dc converter should be
designed based on the off-hook current and off-hook
2 × NREN × VTR_PK
-----------------------------------------------------
=
2
π
--
IAVG = IPK
×
7000π
V
. However, the requirement for the switching
BAT
Equation 3
components (Q7 and Q8 or M1) should still be based on
the V value during ringing.
BAT
The total output power required during ringing is equal
to the power consumed in the load plus the power
consumed in the sensing resistors and the external
transistors of the linefeed circuitry. This leakage current
has a magnitude of 2.5 mA.
Power Input Requirement
The input power is equal to the output power plus the
wasted power during the power conversion process.
The efficiency of the Si321x dc-dc converter is mainly
dependent on the inductor loss (copper and magnetic
loss) and the switching loss. For worst-case estimation,
the efficiency is assumed to be 60% for the
BJT/inductor solution (the actual efficiency is between
63% and 73%) and 75% for the MOSFET/transformer
solution (the actual efficiency is between 75% and
83%.)
POUT = VBAT × (IAVG + 0.0025)
Equation 4
In the Off-Hook State
In a few special cases, the power consumed during
off-hook is higher than the power consumed during
ringing. It is important to check for this and design the
power supply to handle the larger power requirement.
Most designers can skip this section unless their
designs support long line and/or the brief on-hook
voltage measurement (for caller ID 2 and 2.5) with
TRACK = 0 (bit 0 of the direct Register 66).
PO = PIN × Power Efficiency = IIN × VIN × Power Efficiency
Solving for I :
IN
PO
----------------------------------------------------------------
(VIN × Power Efficiency)
IIN
=
The output current equations in the off-hook active state
are as follows:
Equation 8
The input voltage to the dc-dc converter could drop
quickly (depending on the source impedance) as the
2
Rev. 0.5
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