CPC7691 Datasheet, PDF (11/16 Pages) Clare, Inc. – Line Card Access Switch

INTEGRATED CIRCUITS DIVISION
Under all start-up situations the CPC7691 will
condition the internal switch control logic for the All-Off
state thereby ensuring all of the switches will remain
off during power-up. When V_DD requirements have
been satisfied the LCAS will complete it’s start-up
procedure in one of three conditions.
For start-up scenario 1, the CPC7691 will transition
from the All-Off state to the state defined by the
IN_RINGING input when V_DD is valid.
For start-up scenarios 2, 3, 5, and 6, the CPC7691 will
power up in the All-Off state and remain there until the
LATCH pin is pulled low. This allows for an indefinite
All-Off state for boards inserted into a powered system
but are not configured for service or boards that need
to wait for other devices to be configured first.
Scenario 4 will start up with all switches in the All-Off
state, but upon the acceptance of a valid V_DD the
LCAS will revert to either the Talk state or the Ringing
state, and thereafter may randomly change states
based on input pin leakage currents and loading. This
start-up condition should never be utilized as the
LCAS state after power-up can not be predicted.
For the start-up scenario when T_SD = 0 the CPC7691
will condition the internal control logic for the All-Off
state as it does when T_SD  0. Start-up behavior with
an initial T_SD = 0 is dependent on when T_SD is
released with respect to the status of the Under
Voltage Lock Out circuitry. Releasing T_SD (which
allows the internal pull-up to raise the input from a
logic level low to a logic level high before, or
concurrent with, the Under Voltage Lock Out
deactivation) has no effect on the start-up behavior,
and the start-up scenarios previously described are
applicable.
With T_SD held low beyond the deactivation of the
internal Under Voltage Lock Out control, the device will
remain in the All-Off state regardless of the levels on
the IN_RINGING or LATCH inputs. However, it is
important to note that the LATCH is fully functional
once the Under Voltage Lock Out deactivates. This
allows the LATCH to be preconfigured to the state
desired with the release of T_SD.
CPC7691
2.6 V_BAT Pin
Although battery power is not used for switch control, it
is required to direct negative potential faults away from
the SLIC. Because the CPC7691 requires V_BAT to
protect the SLIC from negative potential faults, the
CPC7691 will deactivate and enter the All-Off state
whenever V_BAT is unavailable.
2.6.1 Protection
In the presence of a negative potential fault the
CPC7691BA will draw current from V_BAT to supply
trigger current for the internal integrated protection
circuitry SCR. This integrated SCR is designed to
activate whenever the voltage at T_BAT or R_BAT drops
2V to 4V below the applied voltage on the V_BAT pin.
Because the battery supply at this pin is required to
source trigger current during negative over-voltage
fault conditions at tip and ring, it is important that the
net supplying this current be a low impedance path for
high speed transients such as lightning. This will
permit trigger currents to flow enabling the SCR to
activate, and thereby prevent a fault induced negative
over-voltage event at the T_BAT or R_BAT nodes.
Although the CPC7691BB does not have the SCR for
negative potential fault protection, it utilizes a power
switching diode from each of the T_BAT and R_BAT nodes
to V_BAT, which forward conduct in the presence of
potentials at these nodes more negative than V_BAT.
Proper protection requires V_BAT to supply sufficient
current to counter the transient fault currents restricted
by the current limit functions of the break switches.
2.6.2 Battery Voltage Monitor
The CPC7691 also uses the V_BAT pin to monitor the
battery voltage. If system battery voltage is lost, the
CPC7691 immediately enters the All-Off state, and
remains in this state until the battery voltage is
restored. An internal detector monitoring the system
battery voltage forces the All-Off state anytime the
battery voltage goes more positive than –10V. The
All-Off state is maintained until the battery voltage
goes more negative than –15 V. This battery monitor
feature draws a small current from V_BAT (typically
<1A) and will add slightly to the device’s overall
power dissipation.
R01
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