Specifications
AN93
12 Rev. 0.8
Hookswitch and DCTermination
The hookswitch and dc termination circuitry are shown
in Figure 2 on page 8. 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).
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
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)