Si3220/Si3225
External Unbalanced Ringing
condition when sourcing longer loop lengths, as the
20 Hz ac impedance of an off-hook long loop is
indistinguishable from a heavily-loaded (5 REN) short
loop in the on-hook state. Therefore, a dc ring trip
detection scheme is required when sourcing longer loop
lengths.
The Si3220 can implement either an ac or dc-based ring
trip detection scheme depending on the application. The
Si3225 allows external dc ring trip detection when using
The Si3225 supports centralized, battery-backed
unbalanced ringing schemes by providing a ringing
relay driver as well as inputs from an external ring trip
circuit. Using this scheme, line-card designers can use
the Dual ProSLIC chipset in existing system
architectures with minimal system changes.
Linefeed Overhead Voltage Considerations
During Ringing
a
battery-backed external ringing generator by
The ringing mode output impedance allows ringing
operation without overhead voltage modification
(VOVR = 0). If an offset of the ringing signal from the
ring lead is desired, VOVR can be used for this
purpose.
Ringing Power Considerations
The total power consumption of the Si3220/Si3200 RAM addresses that must be written or monitored to
chipset using internal ringing generation is dependent correctly detect a ring trip condition.
monitoring the ringing feed path through two sensing
inputs on each channel. By monitoring this path, the
Dual ProSLIC detects a dc current flowing in the loop
once the end equipment has gone off-hook. Table 29
provides recommended register and RAM settings for
various applications, and Table 30 lists the register and
on the VDD supply voltage, the desired ringing
amplitude, the total loop impedance, and the ac load
impedance (number of REN). The following equations
can be used to approximate the total current required
for each channel during ringing mode:
Figure 27 illustrates the internal functional blocks that
correctly detect and process a ring trip event. The
primary input to the system is the loop current sense
(ILOOP) value provided by the loop monitoring circuitry
and reported in the ILOOP RAM location register. The
ILOOP RAM location value is processed by the ISP
block when the LFS bits in the linefeed register indicate
the device is in the ringing state. The output of the ISP
then feeds into a pair of programmable, digital, low-pass
filters: one for the ac ring trip detection path and one for
the dc path. The ac path also includes a full-wave
rectifier block prior to the LPF block. The outputs of
each low-pass filter block are then passed on to a
programmable ring trip threshold (RTACTH for ac
detection and RTDCTH for dc detection). Each
threshold block output is then fed to a programmable
debounce filter to ensure a valid ring trip event. The
output of each debounce filter remains constant unless
the input remains in the opposite state for the entire
period of time set using the ac and dc ring trip debounce
interval registers, RTACDB and RTDCDB, respectively.
The outputs of both debounce filter blocks are then
ORed together. If either the ac or the dc ring trip circuits
indicate a valid ring trip event has occurred, the RTP bit
is set. Either the ac or dc ring trip detection circuits are
disabled by setting the respective ring trip threshold
sufficiently high so that it does not trip under any
condition. A ring trip interrupt is also generated if the
RTRIPE bit is enabled.
VRING,PK
---------------------- --
+ IDD,OH
π
2
IDD,AVE
=
×
ZLOOP
VRING,PK
---------------------- --
2
IBAT,AVE
=
×
ZLOOP
π
where:
VRING,PK = VRING,RMS
×
2
ZLOOP = RLOOP + RLOAD + ROUT
R
R
R
= 7000/REN (for North America)
= loop impedance
= ProSLIC output impedance = 320 Ω
LOAD
LOOP
OUT
I
= I overhead current
DD,OH
DD
= 22 mA for V = 3.3 V
DD
= 26 mA for V = 5 V
DD
Ring Trip Detection
A ring trip event signals that the terminal equipment has
transitioned to an off-hook state after ringing has
commenced, ensuring that the ringing signal is removed
before normal speech begins. The Dual ProSLIC is
designed to implement either an ac or dc-based internal
ring trip detection scheme or a combination of both
schemes. The system design is flexible to address
varying loop lengths of different applications. An ac ring
trip detection scheme cannot reliably detect an off-hook
50
Rev. 1.0
SILICONIMAGE [ Silicon image ]
