ATTL7557AAU Ver la hoja de datos (PDF) - Agere -> LSI Corporation
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ATTL7557AAU Datasheet PDF : 28 Pages
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L7556, L7557 Low-Power SLICs
with Battery Switch
Data Sheet
January 2000
Applications (continued)
dc Applications
Battery Feed
The dc feed characteristic can be described by:
IL = -----V---B---A---T-----–----V---O---H----
RL + 2RP + Rdc
VT ⁄ R = (----V----B---A--T------–---V----O---H---)---×----R-----L
RL + 2RP + Rdc
where:
IL = dc loop current.
VT/R = dc loop voltage.
|VBAT| = battery voltage magnitude applied to the power
amplifier stage (VBAT1 or VBAT2).
VOH = overhead voltage. This is the difference between
the battery voltage and the open loop tip/ring
voltage.
RL = loop resistance, not including protection resistors.
RP = protection resistor value.
Rdc = SLIC internal dc feed resistance.
The design begins by drawing the desired dc template.
An example is shown in Figure 25.
50
40
30
1
10 kΩ
20
10
ILIM
–1
Rdc1
0
0
10
20
30
40
50
Notes:
VBAT1 = –48 V.
VBAT2 = –28 V.
ILIM = 22 mA.
Rdc1 = 115 Ω.
LOOP VOLTAGE (V)
12-3050.f (F)
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:
20
Region 1; On-hook and low loop currents. The slope
corresponds to the dc resistance of the SLIC, RDC1
(default is 115 Ω typical). The open circuit voltage is the
battery voltage less the overhead voltage of the device,
VOH (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 RPROG. This
region of the dc template has a high resistance
(10 kΩ).
Calculate the external resistor as follows:
RPROG (kΩ) = 1.67 ILIM (mA)
Switching the Battery
The L7556 and L7557 SLICs provide an input for an
auxiliary battery. Called VBAT2, this power supply
should be lower in magnitude than the primary battery,
VBAT1. Under an acceptable loop condition, VBAT2 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 VBAT1 or VBAT2.
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 Ω, consider the L7556. The
minimum VBAT2 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 VBAT2 as well as VBAT1.
Consider an off-hook threshold of 10 mA. This could
represent a 1000 Ω loop with a 48 V VBAT1 active or a
2000 Ω loop with a 28 V VBAT2 active. In this case, if
the loop is below 1000 Ω or above 2000 Ω, off-hook
detection will be accurate. Between 1000 Ω and
2000 Ω, the detector is battery-dependent. This condi-
tion must be avoided. In our example, since the maxi-
mum loop is 530 Ω, 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
VBAT2 to switch without affecting the loop current. In
this case, the loop range is determined by VBAT1, and
VBAT2 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 VBAT2 switch threshold.
Lucent Technologies Inc.
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