Si3232
Preliminary Rev. 0.96
21
Not
fo
r N
ew
esi
gn
s
Figure 7. DC Linefeed Overhead Voltages
(Forward State)
4.3.1. Calculating Overhead Voltages
The two programmable overhead voltages, VOV and
VCM, represent one portion of the total voltage between
VBAT and ground as illustrated in Figure 7. In normal operating conditions, these overhead voltages are
sufficiently low to maintain the desired TIP-RING
voltage, VOC. 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 VCM
voltage is programmed for a given operating condition.
Therefore,
the
open-circuit
voltage,
VOC, varies
according to the required overhead voltage, VOV, and
the supplied battery voltage, VBAT. The user should pay
special attention to the maximum VOV and VCM that
might be required for each operating state.
In the off-hook active state, sufficient VOC 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.
VCM and VOV are provided in Table 8. The default VCM value of 3 V provides sufficient
overhead for a 3.1 dBm signal into a 600
loop
impedance.
A VOV value of 4 V provides sufficient headroom to
source a maximum ILOOP of 45 mA along with a
3.1 dBm audio signal and an ABIAS setting of 16 mA.
For a typical operating condition, VBAT = –56 V and
ILIM =22mA:
VOC,MAX =56V – (3V + 4V)=49V
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 VOV and VCM must be increased by a voltage
equal to RPROT x ILIM where RPROT is the value of each
protection resistor. Other safety precautions may apply.
overhead voltage during the ringing state.
The Si3232 uses both voltage and current information to
control TIP and RING. Sense resistor RDC (see
Figure 6) measures dc line voltages on TIP and RING;
capacitor CAC 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. RDC and RBAT provide
these
measuring
points.
The
sense
circuitry
is
calibrated on-chip to ensure measurement accuracy.
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
++