I
NTEGRATED
C
IRCUITS
D
IVISION
16 www.ixysic.com R04
CPC7594
2.4 Data Latch
The CPC7594 has an integrated transparent data
latch. The latch enable operation is controlled by TTL
logic input levels at the LATCH pin. Data input to the
latch is via the input pins IN
RINGING
and IN
TEST
while
the output of the data latch are internal nodes used for
state control. When the LATCH enable control pin is at
a logic 0 the data latch is transparent and the input
control signals flow directly through the data latch to
the state control circuitry. A change in input will be
reflected by a change in the switch state.
Whenever the LATCH enable control pin is at logic 1,
the data latch is active and data is locked. Subsequent
changes to the input controls IN
RINGING
and IN
TEST
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 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 having a nominal value
of 16 A 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
CPC7594 will enter thermal shutdown and a logic low
will be output.
As an input, the T
SD
pin is utilized to place the
CPC7594 into the “All-Off” state by simply pulling the
input to a logic low. For applications using low-voltage
logic devices (lower than V
DD
), IXYS IC Division
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, IXYS IC Division recommends all
applications use an open-collector or open-drain type
device to drive this pin.
Unlike the CPC7584, driving T
SD
to a logic 1 or tying
this pin to V
CC
will not prevent normal operation of the
thermal shutdown circuitry inside the CPC7594. As a
result the T
SD
pin may be held at a logic high.
However, the CPC7594 T
SD
pin has only two
recommended operating states when it is used as an
input control. A logic 0, which forces the device to the
all-off state and a high impedance (Z) state for normal
operation. This requires the use of an open-collector
or open-drain type buffer.
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 IXYS IC Division’s 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 in series with the ringing
generator is recommended.
2.7 Power Supplies
Both a +5V supply and battery voltage are connected
to the CPC7594. Switch state control is powered
exclusively by the +5V supply. As a result, the
CPC7594 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
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 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.
