Si3230
Preliminary Rev. 0.96
31
Not
Recommended
fo
r N
ew
D
esi
gn
s
amplitude, where rise time and period are related
through the following equation for the crest factor of a
trapezoidal waveform.
where T = ringing period, and CF = desired crest factor.
For example, to generate a 71 VPK, 20 Hz ringing
signal, the equations are as follows:
For a crest factor of 1.3 and a period of 0.05 seconds
(20 Hz), the rise time requirement is 0.0153 seconds.
In addition, the user must select the trapezoidal ringing
waveform by writing TSWS = 1 in direct Register 34.
2.4.4. Ringing DC voltage Offset
A dc offset can be added to the ac ringing waveform by
defining
the
offset
voltage
in
ROFF
(indirect
Register 19). The offset, VROFF, is added to the ringing
signal when RVO is set to 1 (direct Register 34, bit 1).
The value of ROFF is calculated as follows:
2.4.5. Linefeed Considerations During Ringing
Care must be taken to keep the generated ringing signal
within the ringing voltage rails (GNDA and VBAT) to
maintains
proper
biasing
of
the
external
bipolar
transistors. If the ringing signal nears the rails, a
distorted ringing signal and excessive power dissipation
in the external transistors will result.
To prevent this invalid operation, set the VBATH value
(direct Register 74) to a value higher than the maximum
peak ringing voltage. The discussion below outlines the
considerations and equations that govern the selection
of the VBATH setting for a particular desired peak
ringing voltage.
First, the required amount of ringing overhead voltage,
VOVR, is calculated based on the maximum value of
current through the load, ILOAD,PK, the minimum current
gain of Q5 and Q6, and a reasonable voltage required
to keep Q5 and Q6 out of saturation. For ringing signals
up to VPK =87V, VOVR = 7.5 V is a safe value.
However, to determine VOVR for a specific case, use the
equations below.
where:
NREN is the ringing REN load (max value = 5),
IOS is the offset current flowing in the line driver circuit
(max value = 2 mA), and
VAC,PK = amplitude of the ac ringing waveform.
It is good practice to provide a buffer of a few more
milliamperes for ILOAD,PK to account for possible line
leakages, etc. The total ILOAD,PK current should be
smaller than 80 mA.
where
is the minimum expected current gain of
transistors Q5 and Q6.
The minimum value for VBATH is therefore given by the
following:
The ProSLIC is designed to create a fully balanced
ringing waveform, meaning that the TIP and RING
common mode voltage, (VTIP + VRING)/2, is fixed. This
voltage
is
referred
to
as
VCM_RING
and
is
automatically set to the following:
VCMR is an indirect register which provides the
headroom by the ringing waveform with respect to the
VBATH rail. The value is set as a 4-bit setting in indirect
Register 40
with
an
LSB
voltage
of
1.5 V/LSB.
Register 40 should be set with the calculated VOVR to
provide voltage headroom during ringing.
Silicon revisions C and higher support the option to
briefly increase the maximum differential current limit
between the voltage transition of TIP and RING from
ringing to a dc linefeed state. This mode is enabled by
setting ILIMEN = 1 (direct Register 108, bit 7).
2.4.6. Ring Trip Detection
A ring trip event signals that the terminal equipment has
gone off-hook during the ringing state. The ProSLIC
performs ring trip detection digitally using its on-chip
monitor A/D converter. The functional blocks required to
implement ring trip detection is shown in
Figure 14. The
primary input to the system is the Loop Current Sense
value provided by the current monitoring circuitry and
reported in direct Register 79. LCS data is processed by
the input signal processor when the ProSLIC is in the
t
RISE
3
4
---T1
1
CF
2
-----------
–
=
RNGY 20 Hz
1
2
---
1
20 Hz
----------------
8000
200
C8h
=
RNGX 71 V
PK
71
96
------
2
15
24235
5EABh
=
RCO 20 Hz, 1.3 crest factor
2
24235
0.0153
8000
--------------------------------------
396
018Ch
=
ROFF
V
ROFF
96
------------------
2
15
=
I
LOAD,PK
V
AC,PK
R
LOAD
-------------------
I
OS
+
V
AC,PK
N
REN
6.9 k
------------------
I
OS
+
==
V
OVR
I
LOAD,PK
1
+
-------------
80.6
1V
+
=
VBATH
V
AC,PK
V
ROFF
V
OVR
++
=
VCM_RING
VBATH
VCMR
–
2
----------------------------------------------
=