參數(shù)資料
型號(hào)： L7557
廠商： Lineage Power
英文描述： Low-Power SLICs （electronic subscriber loop interface circuits ）with Battery Switch(帶電池開(kāi)關(guān)的低功耗用戶環(huán)路線接口電路（SLIC）)
中文描述： 低功耗SLIC組件（電子用戶環(huán)路接口電路）與電池開(kāi)關(guān)（帶電池開(kāi)關(guān)的低功耗用戶環(huán)路線接口電路電路（SLIC））
文件頁(yè)數(shù)： 20/28頁(yè)
文件大?。?/td> 753K
代理商： L7557
20
Lucent Technologies Inc.
Data Sheet
January 2000
with Battery Switch
L7556, L7557 Low-Power SLICs
Applications
(continued)
dc Applications
Battery Feed
The dc feed characteristic can be described by:
where:
I
L
= dc loop current.
V
T/R
= dc loop voltage.
|V
BAT
| = battery voltage magnitude applied to the power
amplifier stage (V
BAT1
or V
BAT2
).
V
OH
= overhead voltage. This is the difference between
the battery voltage and the open loop tip/ring
voltage.
R
L
= loop resistance, not including protection resistors.
R
P
= protection resistor value.
R
dc
= SLIC internal dc feed resistance.
The design begins by drawing the desired dc template.
An example is shown in Figure 25.
12-3050.f (F)
Notes:
V
BAT1
= –48 V.
V
BAT2
= –28 V.
I
LIM
= 22 mA.
R
dc1
= 115
.
Figure 25. Loop Current vs. Loop Voltage
Starting from the on-hook condition and going through
to a short circuit, the curve passes through two regions:
Region 1; On-hook and low loop currents. The slope
corresponds to the dc resistance of the SLIC, R
DC1
(default is 115
typical). The open circuit voltage is the
battery voltage less the overhead voltage of the device,
V
OH
(default is 7.9 V typical). These values are suitable
for most applications, but can be adjusted if needed.
For more information, see the sections entitled Adjust-
ing dc Feed Resistance or Adjusting Overhead Volt-
age.
Region 2; Current limit. The dc current is limited to a
value determined by external resistor R
PROG
. This
region of the dc template has a high resistance
(10 k
).
Calculate the external resistor as follows:
R
PROG
(k
) = 1.67 I
LIM
(mA)
Switching the Battery
The L7556 and L7557 SLICs provide an input for an
auxiliary battery. Called V
BAT2
, this power supply
should be lower in magnitude than the primary battery,
V
BAT1
. Under an acceptable loop condition, V
BAT2
can
be switched to provide the loop power through the out-
put amplifiers of the SLIC. The dc template, described
in the last section, is determined by the battery that is
activated—either V
BAT1
or V
BAT2
.
Which device will be best for you That mainly
depends on your loop range requirements. If you have
only short loops and no on-hook voltage requirements,
you don't need a battery switch at all. Use the L7551
instead. If you have only to guarantee a short loop
range, e.g., 22 mA into 530
, consider the L7556. The
minimum V
BAT2
can be determined by the standard dc
equations.
In these applications, the off-hook detector can be
used to indicate when to switch the battery. Just make
sure the off-hook detector will also function as required
with V
BAT2
as well as V
BAT1
.
Consider an off-hook threshold of 10 mA. This could
represent a 1000
loop with a 48 V V
BAT1
active or a
2000
loop with a 28 V V
BAT2
active. In this case, if
the loop is below 1000
or above 2000
, off-hook
detection will be accurate. Between 1000
and
2000
, the detector is battery-dependent. This condi-
tion must be avoided. In our example, since the maxi-
mum loop is 530
, the 10 mA detector is perfectly
acceptable.
If the PTT would like a short loop system that can also
serve long loops, the off-hook detector is not the best
indicator, and better loop intelligence is needed. In this
case, the L7557 can be used. It has an internal com-
parator that senses when there is enough potential at
V
BAT2
to switch without affecting the loop current. In
this case, the loop range is determined by V
BAT1
, and
V
BAT2
is only switched in when the loop is short enough
to use it. This switching is automatic and includes hys-
teresis to avoid oscillation when the loop length is close
to the V
BAT2
switch threshold.
I
L
R
L
+
(
R
L
R
dc
+
2R
P
---------------------------------
=
V
T R
V
BAT
--------------------------------------------
V
OH
–
)
R
dc
R
L
×
2R
P
+
+
=
0
10
20
50
0
20
30
40
50
LOOP VOLTAGE (V)
30
40
10
L
1
10 k
I
LIM
–1
Rdc
1
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