Si3050 + Si3011/18/19
Rev. 1.5
31
CVT[7:0] bits. Therefore, only positive numbers should
be used as a threshold.
5.16. DC Termination
The DAA has programmable settings for the dc
impedance, current limiting, minimum operational loop
current and TIP/RING voltage. The dc impedance of the
DAA is normally represented with a 50
slope as
slope by setting the DCR bit. This higher dc termination
presents a higher resistance to the line as loop current
increases.
Figure 23. FCC Mode I/V Characteristics,
DCV[1:0] = 11, MINI[1:0] = 00, ILIM = 0
For applications requiring current limiting per the TBR21
standard, the ILIM bit may be set to select this mode. In
this mode, the dc I/V curve is changed to a 2000
slope above 40 mA, as shown in
Figure 24. This allows
the DAA to operate with a 50 V, 230
feed, which is the
maximum linefeed specified in the TBR21 standard.
Figure 24. TBR21 Mode I/V Characteristics,
DCV[1:0] = 11, MINI[1:0] = 00, ILIM = 1
The MINI[1:0] bits select the minimum operational loop
current for the DAA, and the DCV[1:0] bits adjust the
DCT pin voltage, which affects the TIP/RING voltage of
the DAA. These bits allow important trade-offs to be
made
between
signal
headroom
and
minimum
operational loop current. Increasing TIP/RING voltage
increases signal headroom, whereas decreasing the
TIP/RING voltage allows compliance to PTT standards
in low-voltage countries, such as Japan. Increasing the
minimum operational loop current above 10 mA also
increases signal headroom and prevents degradation of
the signal level in low-voltage countries.
Finally,
Australia
has
separate
dc
termination
requirements for line seizure versus line hold. Japan
mode (only available with the Si3018 or Si3019) may be
used to satisfy both requirements. However, if a higher
transmit level for modem operation is desired, switch to
FCC mode 500 ms after the initial off-hook. This
satisfies the Australian dc termination requirements.
5.17. AC Termination
The
Si3050
+
Si3011
chipset
provides
two
ac
termination impedances. The Si3050 + Si3018 chipset
provides
four
ac
termination
impedances.
The
ACIM[3:0] bits in Register 30 are used to select the ac
impedance setting. The two available settings for the
Si3050 + Si3011 chipset are listed in
Table 16. The four
available settings for the Si3018 are listed in
Table 17. If
an ACIM[3:0] setting other than the four listed in
forced to 600
(ACIM[3:0] = 0000). The programmable
digital hybrid can be used to further reduce near-end
echo for each of the four listed ac termination settings.
12
11
10
9
8
7
6
.01 .02 .03 .04 .05 .06 .07 .08 .09 .1 .11
Loop Current (A)
FCC DCT Mode
Voltage
Ac
ro
s
DAA
(
V
)
45
40
35
30
25
20
15
10
5
.015 .02 .025 .03 .035 .04 .045 .05 .055 .06
Loop Current (A)
TBR21 DCT Mode
Voltage
Ac
ro
s
DAA
(
V
)
Table 16. AC Termination Settings for the
Si3011 Line-Side Device
ACIM[3:0]
AC Termination
0000
600
0001
210
+ (750 || 150 nF) and 275 +
(780
|| 150 nF)