PBL38772 데이터 시트보기 (PDF) - Ericsson
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PBL38772 Datasheet PDF : 24 Pages
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PBL 387 72
Battery Feed and Automatic Battery
Switching
To reduce short loop power dissipation a second lower
battery voltage, Off-hook or Talk battery, must be connected
to the device via an external diode at terminal VTBAT. The
SLIC automatically switches between the two battery supply
voltages without need for external control. The silent battery
switching to VBAT occurs when the line current is below 5.5
mA. This means that the current in On-hook, VBAT, battery
is limited to 6 mA in the Active state. The On-hook voltage
is derived from VBAT with the range of -43 V to -56 V at the
TIPX and RINGX wires @ VBat ≥ -48 V.
Constant current feed region (figure 12, curve segment
A-B-C)
For TIPX to RINGX voltages VTR < |VTB| – 5.7 V
where
VTR = the tip to ring dc voltage
VTB = the talk battery voltage
5.7 V = the voltage drop from VTB to the line voltage
at point C in the graph of figure 12, calculated
according to:
0.7 V + 3.7 V + (27 mA × 2 × 25 Ω) ≈ 5.7 V
The PBL 387 72/1 emulates constant current loop feed. The
constant current value is adjustable between 18 mA and 30
mA by setting a value for resistor RLC:
RLC
=
500
ILProg
-
10.4
× ln(32
ILProg
×
ILProg)
which may be approximated by
RLC
≈
500
ILProg
where
ILProg desired constant current in A
RLC programming resistance in Ω
ln( ) natural logarithm
Resistive feed region (figure 12, curve segment C-D-E)
For VTR > VTB -5.7 V the PBL 387 72/1 emulates resistive
loop feed with feed resistance equal to 2x25 Ω. The slope of
the resistive feed region is made steep to extend the constant
current region as close to the talk battery voltage (VTBat) as
possible.
On-hook region (figure 12, curve segment E-G-H-J)
For loop currents IL < 5.5 mA the PBL 387 72/1 automatically
switches to feed loop current from the ring battery, VBat.
The switch from talk battery, VTBat, to ring battery, VBat,
occurs without hysteresis at point E in figure 12. For loop
currents IL within the on-hook range 0 mA < IL < 5.0mA
(curve segment G-H-J) the line voltage remains nearly
constant. This feature maintains a high on-hook voltage in the
presence of dc line leakage currents or when a
subscriber device consumes some current from the battery
feed, e.g. to power displays. The On-hook voltage tracks the
VBAT voltage up to 54.5 V, VTROpen = VBat – 4.5 V. For
VBAT higher than 54.5 V the On-hook voltage is limited to
50 V typical.
In the presence of leakage currents ILLk < 5 mA during
on-hook: (Figure 12, curve segment G-H-J)
VTROn-hook = VTROpen - ILLk × RFeed where RFeed = 2×25 Ω
Optimizing VTB
To optimize VTB with actual load on the line:
VTB = (RLMax + RFEED + 2RF) × ILprog + VF + 3.7
where:
RLmax is the maximum loop length including On-hook
phone load
RFeed 2×25 Ω
RF
is the resistance of one fuse resistor.
ILProg is the programmed line current
VF
is the forward voltage of DTB Normal value is 0.7 V
Example: RLmax = 600 Ω, RF = 40 Ω, ILProg = 26.8 mA
This will give a VTB of 24 V.
Silent Polarity Reversal
Polarity reversal time.
The reversal time is set by a capacitor, CSPR, connected
between the pin SPR and AGND. The silent polarity reversal
time is the same in both directions. To calculate the silent
polarity reversal time use the following formula:
tr =CSPR × 9500
The reversal time is measured between the 10% and 90%
values of the line voltage. The reversal time is independent of
line load and line current.
Polarity reversal set-up time.
The set-up time is defined as the time from setting the C1, C2
and C3 inputs to the reversal state until the reversal actually
commences on TIPX and RINGX.
The polarity reversal set-up time is different in the two
directions, active to reversal state and reversal to active state.
The set-up time is calculated from the following expressions:
Active normal state to active reversal state:
tAct→Rev = CSPR × 17500
Active reversal state to active normal state:
tRev→Act = CSPR × 15500
EN/LZT 146 136 R1A © Ericsson Microelectronics, December 2001
15
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