CPC7524

12

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R02

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.

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 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

125°C, placing the channel’s switch pair into the All-Off

state regardless of logic input. During thermal

with a nominal 0V level. A logic high is output from the

If presented with a short-duration transient, such as a

lightning event, the thermal-shutdown feature will

typically not activate. But in an extended

low-frequency 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.

3.6 External Protection Elements

The CPC7524 requires only over-voltage secondary

protection on the high-voltage side of the switch.

Additional external protection may be required on the

low-voltage side of the switch if the threshold of the

high-voltage side protector exceeds the safe operation

of the low-voltage side components. Because the fault

current seen by the low-voltage side protector is

limited by the switch’s active current limit circuitry, the

low-voltage side protector need not be as capable as

that of the high-voltage side protector. The

high-voltage side protector must limit voltage

transients to levels that do not exceed the breakdown

voltage or input-output isolation barrier of the

CPC7524.

3.7 Thermal Design Assessment

A successful design utilizing the CPC7524 Quad High

Voltage Analog Switch Array is dependent on careful

consideration of the application’s environment and the

device’s thermal constraints. For matters regarding the

electrical design, this is simply a case of following the

parameters provided in the preceding tables and for

many this will be sufficient. However, those designers

wishing to push the operational limits envelope with

I

SW

V

SW

V

MAX

I

LIM1

-I

LIM1

2/3 R

ON

-I

LIM2

I

LIM2

-V

MAX

-1.5V

1.5V

R

ON