I
NTEGRATED
C
IRCUITS
D
IVISION
14 www.ixysic.com R01
CPC7691
of switching power diodes connected to F
GND
, one
from T
BAT
and the other from R
BAT
, oriented to forward
conduct whenever the potential at either T
BAT
or R
BAT
becomes positive with respect to F
GND
.
For negative potential fault situations, the CPC7691BA
utilizes a voltage fold-back SCR to direct fault currents
to F
GND
, away from the external devices, while the
CPC7691BB uses a pair of switching power diodes to
direct the fault currents to V
BAT
.
The SCR protection technique of the CPC7691BA
uses the V
BAT
supply as a reference and as a current
source for the SCR trigger to activate the SCR. During
a negative transient event resulting in a bias at T
BAT
or
R
BAT
of 2V to 4V more negative than V
BAT
, the SCR
conducts and faults are shunted to F
GND
. In order for
the SCR to crowbar (or foldback), the SCR’s
on-voltage (see “Protection Circuitry Electrical
Specifications” on page 8) must be less than the applied
voltage at the V
BAT
pin. If the V
BAT
voltage is less
negative than the SCR on-voltage, or if the V
BAT
supply is unable to source the trigger current, the SCR
will not crowbar.
Unlike the CPC7691BA which uses one-half of a
diode bridge and an SCR for the internal protection,
the CPC7691BB instead uses a full diode bridge to
clamp both polarities of a transient fault. In the
CPC7691BB a pair of power switching diodes are
used to direct excessive negative potentials at the
T
BAT
and R
BAT
nodes into V
BAT
. These diodes are
oriented so that anytime the potentials at the T
BAT
or
R
BAT
nodes become more negative than V
BAT
they
forward conduct.
Because these diodes direct negative potential faults
to the V
BAT
pin, the battery supply must be capable of
sourcing currents equal in magnitude to the total
currents of all simultaneous faults. As mentioned
earlier, the fault currents are restricted by the current
limits of the break switches.
Use of the CPC7691BA is recommended for
applications where the local V
BAT
supply is incapable
of sourcing the cumulative transient current demands
of simultaneous multiple port faults.
2.10.3 Thermal Shutdown
The thermal shutdown mechanism activates when the
device die temperature reaches a minimum of 110C,
placing the device in the All-Off state regardless of
logic input. During thermal shutdown events the T
SD
pin will output a logic low with a nominal 0V level. A
logic high is output from the T
SD
pin during normal
operation with a typical output level equal to V
DD
.
When presented with short duration transients such
as a lightning event, the thermal shutdown feature will
typically not activate. But in an extended power-cross
event, fault currents limited by the DC current limit
function flowing through the active switches will cause
the device temperature to rise until the thermal
shutdown mechanism activates. Activation of the
thermal shutdown mechanism forces the CPC7691
into the All-Off state. At this point the current
measured into T
LINE
or R
LINE
will drop to zero. Once
the device enters thermal shutdown it will remain in
the All-Off state until the device temperature drops
below the de-activation level of the thermal shutdown
circuit. This permits the device to autonomously return
to normal operation.
If the fault condition has not passed, the limited current
will again flow through the active switches and heating
will resume, reactivating the thermal shutdown
mechanism. This cycle of entering and exiting the
thermal shutdown mode will continue as long as the
fault condition persists. If the magnitude of the fault
condition is great enough, the external secondary
protector will activate shunting the fault current to
ground.
2.11 External Protection Elements
The CPC7691 requires only over-voltage secondary
protection on the loop (line) side of the device. The
integrated protection feature described above negates
the need for additional external protection on the SLIC
side. The secondary protector must limit voltage
transients to levels that do not exceed the breakdown
voltage or input-output isolation barrier of the
CPC7691. A foldback or crowbar type protector is
recommended to minimize stresses on the switches.
Consult IXYS Integrated Circuits Division’s application
note, AN-144, Impulse Noise Benefits of Line Card Access
Switches for equations related to the specifications of
external secondary protectors, fuse resistors and
PTCs.
