SI3220 Datasheet, PDF (35/112 Pages) Silicon Laboratories – DUAL PROSLIC® PROGRAMMABLE CMOS SLIC/CODEC

Si3220/25
In the case of the discrete bipolar linefeed, since the
source impedance is 320 Ω both before and after the
adaptive linefeed transition, the V/I curve exhibits no
discontinuity at the transition points when
VOCDELTA = 0.
3.4.3. Off-Hook to On-Hook Transition
Load lines of 10 kΩ, 1930 Ω, and 1800 Ω are shown in
Figure 16. These load lines intercept the linefeed V/I
curve at the V/I point that would result if a load of that
resistance value were connected across TIP and RING.
As the dc loop is opened, the dc feed will exit the
constant current region (point 4) and enter the 320 Ω
source impedance region. As the current in the loop
collapses, the VTIP/RING voltage will linearly increase
until VOCHTH (point 5) is reached. At this point,
adaptive linefeed will transition to a source impedance
of 640 Ω (320 Ω for discrete bipolar transistor linefeed)
and decrease the VOC voltage by VOCDELTA (point 6).
3.4.4. VOCTRACK and Adaptive Linefeed
Hysteresis
The two thresholds, VOCLTH and VOCHTH, control
adaptive linefeed hysteresis as shown in Figure 16.
VOCTRACK is a RAM location and is the actual open-
circuit voltage that is being fed to the line. VOCTRACK
is dependent on the measured V_BAT voltage. The
behavior of VOCTRACK is as shown in the equation
below. As long as V_BAT is sufficient to supply VOC +
VOV + VCM, VOCTRACK is equal to the programmed
VOC. However, if V_BAT becomes too small to support
VOC + VOV + VCM, then VOCTRACK will track the
battery voltage so that the programmed VOV and VCM
are satisfied at the expense of a reduced VOC voltage.
In the example of Figure 16, therefore,
VOCTRACK = VOC = 48 V.
The following equation describes VOCTRACK behavior:
| V_BAT |≥ V_OC + V_OV + V_CM ⇒ VOCTRACK= VOC
| V_BAT |< V_OC + V_OV + V_CM ⇒ VOCTRACK=| V_BAT | −(V_OV + V_CM)
The values of VOCLTH and VOCHTH are set relative to
VOCTRACK. In the example shown in Figure 16,
VOCLTH is given as –7 V and VOCHTH as +2 V. This
implies that the VOCLTH threshold is located 7 V below
the prevailing value of VOCTRACK, while the VOCHTH
threshold is located 2 V above the prevailing value of
VOCTRACK.
Therefore, the VOCLTH and VOCHTH thresholds will
automatically track the battery voltage along with
VOCTRACK.
In order to provide an adequate level of adaptive
linefeed hysteresis between the on-hook to off-transition
and the off-hook to on-hook transition, VOCLTH is
programmed below VOCTRACK (e.g., –7 V relative to
VOCTRACK), and VOCHTH is programmed above
VOCTRACK (e.g., +2 V above VOCTRACK). Also,
VOCHTH must be less than VOV – 1 V to ensure that a
proper adaptive linefeed transition will occur in a
reduced battery scenario.
3.5. Ground Start Operation
To configure the dc feed for ground start operation, it is
necessary to write the LINEFEED register with the
value corresponding to either TIP-OPEN (LF[2:0] = 011)
or RING-OPEN (LF[2:0] = 111). The TIP-OPEN and
RING-OPEN linefeed modes place the indicated lead in
the OPEN state (>150 kΩ) while the other lead remains
active.
In ground start operation, an off-hook condition is
signaled by the CPE (Customer Premise Equipment) by
connecting the active lead to earth ground.
The active lead presents a 640 Ω source impedance
before the adaptive linefeed transition (320 Ω for a
discrete bipolar linefeed), and a 320 Ω source
impedance after the adaptive linefeed transition, as
shown in Figure 17. As for loop start operation, the
adaptive linefeed transitions are governed by the
contents of the VOCLTH and VOCHTH RAM
addresses.
The OPEN lead presents a high-impedance (>150 kΩ).
Figure 17 illustrates the ground-start VRING/IRING
behavior using VOC = 48 V and I_LIM = 24 mA in the
TIP-OPEN linefeed state. The ground key current
thresholds are programmable via the LONGLOTH and
LONGHITH RAM addresses. The LONGLPF RAM
address provides filtering of the measured longitudinal
currents, and the LONGDBI RAM address provides de-
bouncing. The LONGHI status bit in register LCRRTP
indicates when a ground key event has been detected.
Upon detecting a ground key event, the linefeed
automatically transitions to the FORWARD ACTIVE (if
initially in TIP-OPEN) or REVERSE ACTIVE (if initially
in RING-OPEN). See “3.11. Ground Key Detection” on
page 45 for additional details.
Rev. 1.2
35