CPC7514Z datasheet(12/15 Pages) IXYS

12 / 15 page

INTEGRATED CIRCUITS DIVISION

CPC7514

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

shown in “Figure 1: Switch Low Frequency

Response” on page 12 the low-frequency

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.

MAX

LIM1

-I

LIM1

2/3 R

-I

LIM2

-V

MAX

-V

2-V1

-1.5V

1.5V

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