Data Sheet
January 2000
L7556, L7557 Low-Power SLICs
with Battery Switch
Features
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Description
These electronic subscriber loop interface circuits
(SLICs) are optimized for low power consumption
while providing an extensive set of features.
The SLICs include an auxiliary battery input and a
built-in switch. In short-loop applications, they can be
used in high battery to present a high on-hook volt-
age, and then switched to low battery to reduce off-
hook power.
The SLICs also include a summing node for meter
pulse injection to 2.2 Vrms. A spare, uncommitted op
amp is included for meter pulse filtering.
The switched battery is applied to the power amplifi-
ers of the device. There are two versions. The L7556
has the battery switch completely under processor
control. The L7557 can automatically switch to lower
battery when appropriate and includes hysteresis to
avoid frequent switching. To make the switch silent,
an external capacitor can be added to slow the tran-
sition.
The L7556 is suited for applications serving only
short loops, where a high on-hook voltage is required
for compatibility with preexisting standards.
The L7557 is suited for applications where a full loop
range is needed, but low short-loop power is desired.
It is a much lower-cost solution than a switching reg-
ulator, and also occupies much less PCB area, need-
ing only a battery filter capacitor and a diode for
implementation.
The device is available in a 32-pin PLCC package. It
is built by using a 90 V complementary bipolar inte-
grated circuit (CBIC) process.
Auxiliary input for second battery, and internal
switch to enable its use to save power
Low active power (typical 125 mW during on-hook
transmission)
Supports meter pulse injection
Spare op amp for meter pulse filtering
–16 V to –60 V power supply operation
Distortion-free on-hook transmission
Convenient operating states:
— Forward powerup
— Disconnect (high impedance)
— 2-wire wink (zero loop voltage)
Adjustable supervision functions:
— Off-hook detector with longitudinal rejection
— Ground key detector
— Ring trip detector
Independent, adjustable, dc and ac parameters:
— dc feed resistance
— Loop current limit
— Termination impedance
Thermal protection
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AGERE [ AGERE SYSTEMS ]
