Si3220/Si3225
The Si3220 also can add a dc offset component to the
ringing signal and detect a ring trip event by monitoring
the dc loop current flowing once the terminal equipment
transitions to the off-hook state. Although adding dc
offset reduces the maximum available ringing amplitude
(using the same ringing supply), this method is required
to reliably detect a valid ring trip event when sourcing
longer loop lengths. The dc offset can be programmed
from 0 to 64.32 V in the RINGOF RAM address as
required to produce adequate dc loop current in the off-
hook state. Depending on the loop length and the ring
trip method, the ac or dc ring trip detection circuits are
disabled by setting their respective ring trip thresholds,
RTACTH or RTDCTH, sufficiently high so that they do
not trip under any condition.
VDD
VCC
Si3220/
Si3225
3 V/5 V Relay
(polarized or
non-polarized)
Relay
Driver
Logic
RRDa/b
TRD1a/b
TRD2a/b
GDD
Si3225 Ring Trip Detection
Figure 28. Dual ProSLIC Internal Relay Drive
Circuitry
The Si3225 implements an external ring trip detection
scheme when using a standard battery-backed external
ringing generator. In this application, the centralized
ringing generator produces an unbalanced ringing
signal that is distributed to individual TIP/RING pairs. A
per-channel ringing relay is required to disconnect the
Si3225 from the TIP/RING pair and apply the ringing
signal. By monitoring the ringing feed path across a ring
The internal driver logic and drive circuitry are powered
by the same V supply as the chip’s main V supply
DD
DD
(VDD1–VDD4 pins). When operating external relays
from a V
supply equal to the chip’s V
supply, an
DD
CC
internal diode network provides protection against
overvoltage conditions from flyback spikes when the
relay is opened. Either 3 V or 5 V relays can be used in
the configuration shown in Figure 28, and either
polarized or non-polarized relays are acceptable if the
feed sense resistor (R
in Figure 31 on page 56) in
RING
series with the ringing source, the Si3225 can detect the
dc current path created when the hook switch inside the
terminal equipment closes. The internal ring trip
detection circuitry is identical to that illustrated in
Figure 27. Figure 31 illustrates the typical external ring
trip circuitry required for the Si3225. Because of the
long loop nature of these applications, a dc ring trip
detection scheme is typically used. The user can
disable the ac ring trip detection circuitry by setting the
RTACTH threshold sufficiently high so it does not trip
under any condition.
V
and V
supplies are identical. The input
CC
DD
IN
impedance, R , of the relay driver pins is a constant
11 Ω while sinking less than the maximum rated 85 mA
into the pin.
If the operating voltage of the relay (V ) is higher than
CC
the Dual ProSLIC V supply voltage, an external drive
DD
circuit is required to eliminate leakage from V to V
CC
DD
through the internal protection diode. In this
configuration, a polarized relay will provide optimal
overvoltage
protection
and
minimal
external
Relay Driver Considerations
components. Figure 29 illustrates the required external
drive circuit, and Table 31 provides recommended
The Dual ProSLIC devices include up to three
dedicated relay drivers to drive external ringing and/or
test relays. Test relay drivers TRD1a, TRD1b, TRD2a,
and TRD2b are provided in all product versions, and
ringing relay drivers RRDa and RRDb are included for
the Si3225 only. In most applications, the relay can be
driven directly from the Dual ProSLIC with no external
relay drive circuitry required. Figure 28 illustrates the
internal relay driver circuitry using a 3 V or 5 V relay.
values for R
for typical relay characteristics and V
CC
DRV
supplies. The output impedance, R
, of the relay
OUT
driver pins is a constant 63 Ω while sourcing less than
the maximum rated 28 mA out of the pin.
Rev. 1.0
53
SILICONIMAGE [ Silicon image ]
