CPC7512

12

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R01

3.5.2 Thermal Shutdown

The thermal-shutdown mechanism activates when the

device’s die temperature reaches a minimum of

110°C, placing the device into the All-Off state

regardless of logic input. During thermal shutdown

events the TSD pin will output a logic low with a

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

pin during normal operation with a typical output level

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 device’s

switches to the All-Off state. At this point the current

into the active switch will drop to zero. Once the device

enters thermal shutdown, it will remain in the All-Off

state until the internal temperature of the device 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 and heating will resume,

causing the thermal-shutdown mechanism to

reactivate. 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, with an external

over-voltage protector present, the external protector

will activate shunting the fault current to ground.

3.6 External Protection Elements

The CPC7512 requires only over-voltage 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 high frequency dynamic current

limit, 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

CPC7512. A foldback or crowbar type protector on the

high-voltage side is recommended to minimize

stresses on the CPC7512.

3.7 Thermal Design Assessment

A successful design utilizing the CPC7512

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 higher switch current and/or higher

ambient operating temperatures will need to consider

the thermal performance.

Being a real physical device the CPC7512 has a finite

thermal capability that when properly considered will

ensure appropriate behavior and performance.

Determination of the thermal constraint is easily

accomplished using the following power equations:

and

Where

is the dissipated power drawn from the

is the total power dissipated by

from the “VDD Voltage Supply Specifications” on

page 7 while the power dissipated by the switches is

the sum of the concurrently active switches. Total

switch power is the sum of: the squared maximum

current through each active switch times the

The second equation is used to calculate the

maximum ambient temperature the device can be

operated in based on the calculated total power of the

first equation;

T, the junction temperature rise of the

CPC7512 from ambient; and



impedance of the device package are used to

determine the maximum operating ambient

temperature.

DD

P

SW

+

=

T



JA

---------

=

P

V

DD

P

SW