SI3200-X-FS SILABS [Silicon Laboratories], SI3200-X-FS Datasheet - Page 34

• Manufacturer Part Number: SI3200-X-FS
• Manufacturer: SILABS [Silicon Laboratories]

Si3220/25
When the Si3220/Si3225 is used with the Si3200
linefeed device, the source impedance of the dc feed is
640 Ω before the adaptive linefeed transition and 320 Ω
after the adaptive linefeed transition, as shown in
Figure 16. On the other hand, when the Si3220/Si3225
is used with a discrete bipolar transistor linefeed, the
source impedance of the dc feed is 320 Ω both before
and after the adaptive linefeed transition.
The loop closure thresholds are programmable via the
LCROFFHK and LCRONHK RAM addresses. The
LCRLPF RAM address provides filtering of the
measured loop current, and the LCRDBI RAM address
provides de-bouncing. The LCR status bit in register
LCRRTP indicates when a loop closure event has been
detected. See “3.10. Loop Closure Detection” on
page 44 for additional details.
3.4.2. On-Hook to Off-Hook Transition
Referring to Figure 16, point 1 represents the open-
circuit voltage (VOC = –48 V) of the dc feed. At point 1,
the source impedance of the dc feed is 640 Ω (320 Ω for
discrete
RING = VOC, since no current flows in the loop. When
34
bipolar
transistor
70
60
50
40
30
20
10
0
VOCDELTA
10 kOhms
1
linefeed)
Figure 16. Adaptive Linefeed V/I Behavior
0.005
and
6
2450 Ohms
VTIP/
0.01
Rev. 1.2
Iloop (A)
2
a dc load is connected across TIP and RING and as dc
current begins to flow in the dc loop, the product of the
dc loop current and the 640 Ω source impedance
(320 Ω for a discrete bipolar transistor linefeed) causes
the VTIP/RING voltage to decline linearly with
increasing loop current. When the VTIP/RING voltage
reaches the VOCLTH threshold (point 2), adaptive
linefeed switches the source impedance of the dc feed
to 320 Ω and simultaneously boosts the value of VOC
by VOCDELTA (point 3). The source impedance of the
dc feed will now remain at 320 Ω until the programmed
current limit (ILIM) is reached (point 4). At point 4, the dc
feed has entered into the constant current mode of
operation.
Adaptive linefeed can be disabled by writing a value of
zero into the VOCDELTA RAM address. Writing a value
of zero to VOCDELTA simply eliminates the apparent
VOC voltage boost associated with a non-zero
VOCDELTA value. With VOCDELTA = 0 in the case of
the Si3200, the adaptive linefeed transition still changes
the source impedance from 640 Ω to 320 Ω , and there
is a corresponding discontinuity at the transition point.
1930 Ohms
5
1800 Ohms
3
0.015
320 Ohms
VOCHTH
0.02
VOCLTH
4
0.025