INTEGRATED CIRCUITS DIVISION
CPC7514
12
www.ixysic.com
R06
3.5.2 Low Frequency Current Limit
High-power, low-frequency faults are generally the
result of power cross (also known as power contact)
events. Current through a switch in the ON state
during a power-cross fault will be constrained by the
low-frequency current-limit response of the switch. As
current-limit response is dependent on the differential
voltage across the switch. For low levels of fault
current the graph shows that the voltage across the
active switch increases with increasing fault current.
When the magnitude of the fault current into the
CPC7514 reaches the Current Limit 1 (ILIM1)
threshold, the switch ceases to accept additional
current causing the switch response to transition from
low impedance to high impedance. This causes the
voltage across the switch to increase rapidly.
With a fault source of sufficient magnitude, the voltage
across the switch will continue to increase. To limit
on-chip power dissipation, the switch will decrease the
Current Limit when the voltage across the switch
reaches the Current-Foldback Voltage-Breakpoint 1
(V1) level. Additional increases in switch voltage will
cause the switch to transition to Current Limit 2 (ILIM2)
at Current-Foldback Voltage-Breakpoint 2 (V2) further
reducing the thermal loading of the switch.
Figure 1: Switch Low Frequency Response
Thermal management of each channel is necessary
to minimize the internal temperature rise inside the
package, created by a fault on one channel, from
causing a thermal shutdown event of the other
channel.
It is important to note that the low-frequency
current-limit performance is dependent on a voltage
clamping device on the low-voltage side sized to
ensure that fault voltages do not exceed the
specifications of the low-voltage circuits, and capable
of redirecting currents up to the maximum level of
Current Limit 1.
Note that the current-limit circuitry has a negative
temperature coefficient. As a result, if the device is
subjected to extended heating due to a prolonged
power-cross fault condition, the current through the
active switch will decrease as the device temperature
rises. If the device temperature rises sufficiently, then
the temperature shutdown mechanism will activate
and the channel will enter the All-Off state.
3.5.3 Thermal Shutdown
The thermal-shutdown mechanism activates when the
channel’s die temperature reaches a minimum of
110°C, placing the channel’s switch pair into the All-Off
state regardless of logic input. During thermal
shutdown events the TSDx pin will output a logic low
with a nominal 0V level. A logic high is output from the
TSDx pin during normal operation with a typical output
level equal to VDD.
If presented with a short-duration transient, such as a
lightning event, the thermal-shutdown feature will
typically not activate. But in an extended power-cross
event, the device temperature will rise, and the
thermal shutdown mechanism will activate, forcing the
channel’s switches to the All-Off state. At this point the
current into the active switch will drop to zero. Once
the channel enters thermal shutdown, it will remain in
the All-Off state until the temperature of the channel
drops below the de-activation level of the
thermal-shutdown circuit. This permits the circuit to
autonomously return to normal operation. If the fault
has not passed, current will again flow up to the value
allowed by the low-frequency current-limit of the
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
over-voltage protector will activate, shunting the fault
current to ground.
I
SW
V
SW
V
MAX
I
LIM1
-I
LIM1
2/3 R
ON
-I
LIM2
I
LIM2
-V
MAX
-V
2-V1
-1.5V
V
2
V
1
1.5V
R
ON