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Si3232
Preliminary Rev. 0.96
21
Figure 7. DC Linefeed Overhead Voltages
(Forward State)
4.3.1. Calculating Overhead Voltages
The two programmable overhead voltages, V
OV
and
V
CM
, represent one portion of the total voltage between
V
BAT
and ground as illustrated in Figure 7. In normal
operating conditions, these overhead voltages are
sufficiently low to maintain the desired TIP-RING
voltage, V
OC
. There are, however, certain conditions
under which the user must exercise care in providing a
battery supply with enough amplitude to supply the
required TIP-RING voltage as well as enough margin to
accommodate these overhead voltages. The V
CM
voltage is programmed for a given operating condition.
Therefore, the open-circuit voltage, V
OC
, varies
according to the required overhead voltage, V
OV
, and
the supplied battery voltage, V
BAT
. The user should pay
special attention to the maximum V
OV
and V
CM
that
might be required for each operating state.
In the off-hook active state, sufficient V
OC
must be
maintained to correctly power the phone from the
battery supply that has been provided. Since the battery
supply depends on the state of the input supply (i.e.,
charging, discharging, or battery backup mode), the
user must decide how much loop current is required and
determine the maximum loop impedance that can be
driven based on the battery supply provided.
The minimum battery supply required can be calculated
according to the following equation.
V
CM
and V
OV
are provided in Table 8.
The default V
CM
value of 3 V provides sufficient
overhead for a 3.1 dBm signal into a 600
loop
impedance.
A V
OV
value of 4 V provides sufficient headroom to
source a maximum I
LOOP
of 45 mA along with a
3.1 dBm audio signal and an ABIAS setting of 16 mA.
For a typical operating condition, V
BAT
= –56 V and
I
LIM
= 22 mA:
V
OC,MAX
= 56 V – (3 V + 4 V) = 49 V
These conditions apply when the dc-sensing inputs,
STIPDCa/b and SRINGDCa/b, are placed on the SLIC
side of any protection resistance placed in series with
the TIP and RING leads. If line-side sensing is desired,
both V
OV
and V
CM
must be increased by a voltage
equal to R
PROT
x I
LIM
where R
PROT
is the value of each
protection resistor. Other safety precautions may apply.
See "4.7.3. Linefeed Overhead Voltage Considerations
During Ringing" on page 40 for details on calculating the
overhead voltage during the ringing state.
The Si3232 uses both voltage and current information to
control TIP and RING. Sense resistor R
DC
(see
Figure 6) measures dc line voltages on TIP and RING;
capacitor C
AC
couples the ac line voltages on the TIP
and RING leads to be measured. The Si3232 uses the
Si3200 linefeed interface IC to drive TIP and RING and
to isolate the high-voltage line from the low-voltage
Si3232.
The Si3232 measures voltage at various nodes to
monitor the linefeed current. R
DC
and R
BAT
provide
these measuring points. The sense circuitry is
calibrated on-chip to ensure measurement accuracy.
See "4.4. Linefeed Calibration" on page 25 for details.
Constant I Region
Constant V Region
V
CM
V
OC
V
OV
V
OV
R
LOOP
V
BATH
V
TIP
V
RING
V
BATL
Secondary V
BAT
Selected
V
Loop Closure Threshold
V
BAT
V
OC
V
CM
V
OV
+
+
≥