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| 型號(hào)： | SI3220PPTX-EVB |
| 廠商： | Silicon Laboratories Inc |
| 文件頁數(shù)： | 43/112頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL W/DISCRETE INTERFACE |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 系列： | ProSLIC® |
| 主要目的： | 接口，模擬前端（AFE） |
| 已用 IC / 零件： | Si3220 |
| 已供物品： | 板，CD |
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Si3220/25 Si3200/02
36
Rev. 1.3
Not
Recommended
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transition point. In the case of the discrete bipolar
linefeed, since the source impedance is 320
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 k
, 1930 and 1800 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
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
(320 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 VBAT voltage. The
behavior of VOCTRACK is as shown in the equation
below. As long as VBAT is sufficient to supply VOC +
VOV + VCM, VOCTRACK is equal to the programmed
VOC. However, if VBAT 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
therefore,
VOCTRACK = VOC = 48 V.
The following equation describes VOCTRACK behavior:
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 k
) 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
source impedance
before the adaptive linefeed transition (320
for a
discrete
bipolar
linefeed),
and
a
320
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 k
).
Figure 17 illustrates the ground-start VRING/IRING
behavior using VOC = 48 V and ILIM =24mA 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 46 for additional details.
)
V
(
|
V
|
VOCTRACK
V
|
V
|
VOC
VOCTRACK
V
|
V
|
CM
OV
BAT
CM
OV
OC
BAT
CM
OV
OC
BAT
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