CPC7591 Datasheet, PDF (13/19 Pages) Clare, Inc. – Line Card Access Switch

CPC7591
Whenever the LATCH enable control pin is at logic 1,
the data latch is active and data is locked. Subsequent
IN_RINGING input changes will not result in a change to
the control logic or affect the existing switch state.
The switches will remain in the state they were in
when the LATCH pin changes from logic 0 to logic 1
and will not respond to changes in input as long as the
LATCH is at logic 1. However, neither the T_SD input
nor the T_SD output control functions are affected by
the latch function. Since internal thermal shutdown
control and external “All-off” control is not affected by
the state of the LATCH enable input, T_SD will override
state control.
2.5 T_SD Pin Description
The T_SD pin is a bi-directional I/O structure with an
internal pull-up current source with a nominal value of
16 μA biased from V_DD.
As an output, this pin indicates the status of the
thermal shutdown circuitry. Typically, during normal
operation, this pin will be pulled up to V_DD but under
fault conditions that create excess thermal loading the
CPC7591 will enter thermal shutdown and a logic low
will be output.
As an input, the T_SD pin is utilized to place the
CPC7591 into the “All-Off” state by simply pulling the
input low. For applications using low-voltage logic
devices (lower than V_DD), Clare recommends the use
of an open-collector or an open-drain type output to
control T_SD. This avoids sinking the T_SD pull up bias
current to ground during normal operation when the
all-off state is not required. In general, Clare
recommends all applications use an open-collector or
open-drain type device to drive this pin.
2.6 Ringing Switch Zero-Cross Current Turn Off
After the application of a logic input to turn SW4 off,
the ringing switch is designed to delay the change in
state until the next zero-crossing. Once on, the switch
requires a zero-current cross to turn off, and therefore
should not be used to switch a pure DC signal. The
switch will remain in the on state no matter the logic
input until the next zero crossing. These switching
characteristics will reduce and possibly eliminate
overall system impulse noise normally associated with
ringing switches. See Clare application note AN-144,
Impulse Noise Benefits of Line Card Access Switches for
more information. The attributes of ringing switch SW4
may make it possible to eliminate the need for a
zero-cross switching scheme. A minimum impedance
of 300 Ω in series with the ringing generator is
recommended.
2.7 Power Supplies
Both a +5 V supply and battery voltage are connected
to the CPC7591. Switch state control is powered
exclusively by the +5 V supply. As a result, the
CPC7591 exhibits extremely low power consumption
during active and idle states.
Although battery power is not used for switch control, it
is required to supply trigger current for the integrated
internal protection circuitry SCR during fault
conditions. This integrated SCR is designed to
activate whenever the voltage at T_BAT or R_BAT drops 2
to 4 V below the applied voltage on the V_BAT pin.
Because the battery supply at this pin is required to
source trigger current during negative overvoltage
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
overvoltage event at the T_BAT or R_BAT nodes.
2.8 Battery Voltage Monitor
The CPC7591 also uses the V_BAT pin to monitor
battery voltage. If system battery voltage is lost, the
CPC7591 immediately enters the all-off state. It
remains in this state until the battery voltage is
restored. The device also enters the all-off state if the
system battery voltage goes more positive than –10 V,
and remains in the all-off state until the battery voltage
goes more negative than –15 V. This battery monitor
feature draws a small current from the battery (less
than 1 μA typical) and will add slightly to the device’s
overall power dissipation.
This monitor function performs properly if the
CPC7591 and SLIC share a common battery supply
origin. Otherwise, if battery is lost to the CPC7591 but
not to the SLIC, then the V_BAT pin will be internally
biased by the potential applied at the T_BAT or R_BAT
pins via the internal protection circuitry SCR trigger
current path.
R05
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