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# ISO7740: Asking for the ISO774 design suggestion

Part Number: ISO7740
Other Parts Discussed in Thread: TEST

Hi Team，

My customer is surveying ISO774x and has a question about power rating.

Below table shows that ISO7740 still can handle 200mW power dissipation at Tj=150C, but datasheet also shows the Max. Tj is 150C. So, when Tj=150C any additional power dissipation can trigger device to thermal projection, right? How can I explain it to customer?

Besides, below diagram shows the power limiting is 0mW when ambient temp. is 150C(the Tj is 150C supposedly). These two data are conflict, isn't it?

BR,

SHH

• Hi SHH,

Please note that the junction temperature (Tj) is the temperature of transistor junction in the die inside the device. It represents overall maximum temperature for the die. While ambient temperature (Ta) is the temperature of the surrounding air or environment in which the device resides. The datasheet of ISO7740 specifies a maximum value of 150C for Tj while it is 125C for Ta.

Below table shows that ISO7740 still can handle 200mW power dissipation at Tj=150C, but datasheet also shows the Max. Tj is 150C. So, when Tj=150C any additional power dissipation can trigger device to thermal projection, right? How can I explain it to customer?

The test conditions stated in the table are the worst-case operating conditions and the maximum power dissipation supported is specified to be as 200mW under these worst-case test conditions.
When the device is operated under recommended operating conditions, the power will always remain under 200mW. Since it is a signal only transfer device, it cannot be forced to dissipate excess power without violating device operating conditions. Hence, you do not have to worry about power dissipation of this device as long as device is operated under recommended operating conditions.

Besides, below diagram shows the power limiting is 0mW when ambient temp. is 150C(the Tj is 150C supposedly). These two data are conflict, isn't it?

Please note that the curve shows 0mW when Ta = 150C because the max supported Ta is only 125C and 150C is not supported. The relationship between Tj and Ta can be expressed in terms of thermal resistance, as shown below.

Regards,
Koteshwar Rao

• thanks for support!