Specifications

AN93
Rev. 0.9 15
2.2.11. Hookswitch and DC Termination
The hookswitch and dc termination circuitry are shown
in Figure 2 on page 10. Q1, Q2, Q3, Q4, R5. R6, R7,
R8, R15, R16, R17, R19, and R24 perform the
hookswitch function. The on-hook/off-hook condition of
the modem is controlled by Si3018/10 pins 13 (QB) and
1 (QE).
2.2.11.1. 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 Ω slope as
shown in Figure 5, but can be changed to an 800 Ω
slope by setting the DCR bit. This higher dc termination
presents a higher resistance to the line as loop current
increases.
Figure 5. FCC Mode I/V Characteristics
DCV[1:0]
= 11, MINI[1:0] = 00
For applications requiring current limiting per the legacy
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 Ω slope above 40 mA, as shown in Figure 6. This
allows the DAA to operate with a 50 V, 230 Ω feed,
which is the maximum linefeed specified in the TBR21
standard.
Figure 6. TBR21 Mode I/V Characteristics
DCV[1:0]
= 11, MINI[1:0] = 00
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.
2.2.12. AC Termination
The Si2493/57/34/15/04 has four ac termination
impedances when used with the Si3018 line-side
device. The ACT bits in Register U63 are used to select
the ac impedance setting on the Si3018. The four
available settings for the Si3018 are listed in Table 5. If
an ACT[3:0] setting other than the four listed in Table 5
is selected, the ac termination is forced to 600 Ω
(ACT[3:0] = 0000).
Table 4. Loop Current Transfer Function
LVCS[4:0] Condition
00000 Insufficient line current for normal operation.
00001 Minimum line current for normal operation.
11111 Loop current is excessive (overload). Overload > 128 mA in all modes
except CTR21.
Overload > 56 mA in CTR21 mode.
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 Across 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 Across DAA (V)