CPC7691 Datasheet, PDF (14/16 Pages) Clare, Inc. – Line Card Access Switch

INTEGRATED CIRCUITS DIVISION
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 110C,
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.
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