Si3220/Si3225
Figure 16. Adaptive Linefeed V/I Behavior
When the Si3220/Si3225 is used with the Si3200 the source impedance of the dc feed is 640 Ω (320 Ω for
linefeed device, the source impedance of the dc feed is discrete bipolar transistor linefeed) and VTIP/
640 Ω before the adaptive linefeed transition and 320 Ω RING = VOC, since no current flows in the loop. When
after the adaptive linefeed transition, as shown in a dc load is connected across TIP and RING and as dc
Figure 16. On the other hand, when the Si3220/Si3225 current begins to flow in the dc loop, the product of the
is used with a discrete bipolar transistor linefeed the dc loop current and the 640 Ω source impedance
source impedance of the dc feed is 320 Ω both before (320 Ω for a discrete bipolar transistor linefeed) causes
and after the adaptive linefeed transition.
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.
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 “Loop Closure Detection” on page 42 for
additional details.
On-Hook to Off-Hook Transition
Referring to Figure 16, point 1 represents the open- zero into the VOCDELTA RAM address. Writing a value
circuit voltage (VOC = –48 V) of the dc feed. At point 1, of zero to VOCDELTA simply eliminates the apparent
Adaptive linefeed can be disabled by writing a value of
Rev. 1.0
33
SILICONIMAGE [ Silicon image ]
