參數(shù)資料
型號(hào)： L7556
英文描述： Low-Power SLICs with Battery Switch
中文描述： 低功耗SLIC組件的電池開(kāi)關(guān)
文件頁(yè)數(shù)： 21/28頁(yè)
文件大?。?/td> 700K
代理商： L7556
Lucent Technologies Inc.
21
Data Sheet
January 2000
with Battery Switch
L7556, L7557 Low-Power SLICs
Applications
(continued)
dc Applications
(continued)
Overhead Voltage
In order to drive an on-hook ac signal, the SLIC must
set up the tip and ring voltage to a value less than the
battery voltage. The amount that the open loop voltage
is decreased relative to the battery is referred to as the
overhead voltage. Expressed as an equation:
V
OH
=
|
V
BAT
|
– (V
PT
– V
PR
)
Without this buffer voltage, amplifier saturation will
occur and the signal will be clipped. The device is auto-
matically set at the factory to allow undistorted on-hook
transmission of a 3.17 dBm signal into a 900
loop
impedance. For applications where higher signal levels
are needed, e.g., periodic pulse metering, the 2-wire
port of the SLIC can be programmed with pin DCR.
The drive amplifiers are capable of 4 Vrms minimum
(VAMP). Referring to Figure 26, the internal resistance
has a worst-case value of 46
. So, the maximum sig-
nal the device can guarantee is:
=
Thus, R
P
≤
35
allows 2.2
Vrms
metering signals. The
next step is to determine the amount of overhead volt-
age needed. The peak voltage at output of tip and ring
amplifiers is related to the peak signal voltage by:
12-2560.e (F)
Figure 26
.
SLIC 2-Wire Output Stage
In addition to the required peak signal level, the SLIC
needs about 2 V from each power supply to bias the
amplifier circuitry. It can be thought of as an internal
saturation voltage. Combining the saturation voltage
and the peak signal level, the required overhead can
be expressed as:
where V
SAT
is the combined internal saturation voltage
between the tip/ring amplifiers and V
BAT
(5.4 V typ.).
R
P
(
) is the protection resistor value, and 40
is the
output series resistance of each internal amplifier.
Z
T/R
(
) is the ac loop impedance.
Example 1, on-hook transmission of a meter pulse:
Signal level: 2.2 Vrms into 200
35
protection resistors
I
LOOP
= 0 (on-hook transmission of the metering signal)
V
OH
= 5.4 +
(2.2) = 10.8 V
Accounting for V
SAT
tolerance of 0.5 V, a nominal
undistorted 2.2 V metering signal.
Adjusting Overhead Voltage
To adjust the open loop 2-wire voltage, pin DCR is
programmed at the midpoint of a resistive divider from
ground to either –5 V or V
BAT
. In the case of –5 V, the
overhead voltage will be independent of the battery
voltage. Figure 27 shows the equivalent input circuit to
adjust the overhead voltage.
12-2562 (F)
Figure 27. Equivalent Circuit for Adjusting the
Overhead Voltage
The overhead voltage is programmed by using the fol-
lowing equation:
V
OH
= 7.9 – 4 V
DCR
V
T R
4 V
2 R
P
Z
T/R
46
+
)
+
-----------------------------------------
Vamp = V
T/R
1
-----------------------------
+
)
Z
T R
+
Λ
Λ
R
P
R
P
40
40
V
AMP
–
+
[Z
T/R
]
–
+
V
T/R
V
OH
V
SAT
1
------------------------------
+
)
Z
T R
+
V
T R
+
=
Λ
1
-----------------------------
)
200
+
2
DCR
25 k
± 30%
R1
R2
–5 V
7.9
4
5
||
R
2
R
1
25 k
+
||
-------------------------------------
×
–
–
=
7.9
20
||
R
2
R
1
25 k
+
||
-------------------------------------
+
=
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