Si3220/Si3225
according to the required overhead voltage (V ) and
DC Feed Characteristics
OV
the supplied battery voltage (V
pay attention to the maximum V
). The user should
BAT
OV
The Si3220 and Si3225 offer programmable constant-
voltage and constant-current operating regions as
illustrated in Figure 14 and Figure . The constant
and V
that might
CM
be required for each operating state.
In the off-hook active state, sufficient V
must be
OC
voltage region (defined by the open-circuit voltage, V
)
OC
maintained to correctly power the phone from the
battery supply that is provided. Because 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 with
the following equation:
is programmable from 0 to 63.3 V in 1 V steps. The
constant current region (defined by the loop current
limit, I ) is programmable from 18 to 45 mA in
LIM
0.87 mA steps. The Si3220 and Si3225 exhibit a
characteristic dc impedance of 640 Ω or 320 Ω during
Active mode. (See "Adaptive Linefeed" on page 32).
The TIP-RING voltage (V ) is offset from ground by a
OC
programmable voltage (V ) to provide sufficient
CM
voltage headroom to the most positive terminal
(typically the TIP lead in normal polarity or the RING
lead in reverse polarity) for carrying audio signals. A
V
BAT ≥ VOC + VCM + VOV
similar programmable voltage (V ) is an offset
OV
where V and V are provided in Table 8 on page 13.
CM
OV
CM
between the most negative terminal and the battery
supply rail for carrying audio signals. (See Figure 14.)
The user-supplied battery voltage must have sufficient
amplitude under all operating states to ensure sufficient
headroom. The Si3200 may be powered by a lower
The default V
value of 3 V provides sufficient
overhead for a 3.1 dBm signal into a 600 Ω loop
impedance with an I
setting of 22 mA and an ABIAS
value of 4 V provides sufficient
LIM
OV
setting of 4 mA. A V
headroom to source a maximum I
of 45 mA with a
LOOP
secondary battery supply (V
) to reduce total power
BATL
3.1 dBm audio signal and an ABIAS setting of 16 mA.
For a typical operating condition of V = –56 V and
dissipation when driving short-loop lengths.
BAT
I
= 22 mA:
LIM
Loop Closure Threshold
RLOOP
Constant I Region
Constant V Region
VOC, MAX = 56 V – (3 V + 4 V) = 49 V
VCM
VTIP
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,
VOC
VOV
VBATL
VRING
VBATH
both V
and V
must be increased by a voltage
CM
VOV
OV
Secondary VBAT
Selected
equal to R
x I
where R
is the value of each
V
PROT
LIM
PROT
protection resistor. Other safety precautions may also
apply.
Figure 14. DC Linefeed Overhead Voltages
(Forward State)
See "Linefeed Overhead Voltage Considerations During
Ringing" on page 50 for details on calculating the
overhead voltage during the ringing state.
Calculating Overhead Voltages
The two programmable overhead voltages (V
The Dual ProSLIC chipset uses both voltage and
current information to control TIP and RING. Sense
and
OV
resistor R
measures dc line voltages on TIP and
V
V
) represent one portion of the total voltage between
DC
CM
RING; Capacitor C couples the ac line voltages on
and ground as illustrated in Figure 14. Under
AC
BAT
the TIP and RING leads to be measured. The Si3220
and Si3225 both use the Si3200 to drive TIP and RING
and isolate the high-voltage line from the low-voltage
CMOS devices.
normal operating conditions, these overhead voltages
are sufficiently low to maintain the desired TIP-RING
voltage (V ). However, there are certain conditions
OC
under which the user must exercise care in providing a
battery supply with enough amplitude to supply the
required TIP-RING voltage and enough margin to
The Si3220 and Si3225 measure voltage at various
nodes to monitor the linefeed current. R
and R
BAT
DC
accommodate these overhead voltages. The V
provide these measuring points. The sense circuitry is
calibrated on-chip to guarantee measurement accuracy.
See "Linefeed Calibration" on page 35 for details.
CM
voltage is programmed for a given operating condition.
Therefore, the open-circuit voltage (V
)
varies
OC
Rev. 1.0
29
SILICONIMAGE [ Silicon image ]
