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# SN74HC257: Tj(max) for SN74HC257, CD74HC4075 and SN74HC86

Part Number: SN74HC257
Other Parts Discussed in Thread: CD74HC4075, , SN74HC86

Hi Team

I understood logic's Tj(max) will be typically 150℃ unless otherwise specified in datasheet from following E2E link.

Just want to double-check with you.

Is it correct that all SN74HC257, CD74HC4075 and SN74HC86 have 150℃ Tj(max)? Thanks.

Customer want to compare the maximum power dissipation to other company's logic, so they are trying to use "Pd=（Tj(max)-TA）/θJA" to calculate maximum power dissipation.

Please also let me know if this formula is not correct.

"For logic devices, TJ(max) is equal to the maximum storage temperature (Tstg(max)) as listed in the Absolute Maximum Ratings table of the datasheet, unless otherwise specified. This value is typically 150°C."

https://e2e.ti.com/support/logic-group/logic/f/logic-forum/716186/faq-what-is-the-maximum-junction-temperature-tjmax-for-a-device?tisearch=e2e-sitesearch&keymatch=logic%2525252525252520power%2525252525252520dissipation%2525252525252520calculate#

Thanks.

Regards,

Jo

• Hi Jo,

Yes, all of the information you have is correct.

As an example, at 85C operation, the SN74HC86 in the PW package (113 C/W) can dissipate up to 575 mW.

• Hi Emrys,

My customer used 150C for Tj to calculate maximum power dissipation. I got questions from customer.

From above formula, maximum power dissipation for SN74HC257NSR is 1W and CD74HC4075NSR is 855mW respectively. But Toshiba's maximum power dissipation for TC74HC257AF and TC74HC4075AF is both 180mW(shown in datasheet).

Above are same package(SOP) but the maximum power dissipation are quite different, so customer want to confirm is it correct that SN74HC257NSR and CD74HC4075NSR can really dissipate power up to 1W and 855mW respectively?

Also, SN74HC86N is 812mW, which is also larger than Toshiba's TC74HC86AP(500mW). Can you also confirm this?

If calculation is correct, can you explain the difference why TI's IC can allow more power dissipation?

Thanks.

Regards,

Jo

• Hi Jo,

Out of curiosity, are they planning on trying to drive this much power through the device? This is not a power delivery device, but a logic gate.

Have they considered asking Toshiba why they did this? I can only speak for TI.

180mW is an extremely low value for thermal dissipation. My guess would be that they tested their device at 180mW dissipation, saw that it worked,  and then rated it to that value. It's possible that the device has extremely poor thermal performance - I'm afraid I don't know.  A smaller die and lower leadframe mass will reduce the ability of the device to dissipate heat efficiently.

We use thermal resistance values because it's more accurate for our customers and allows them flexibility in how they use our parts in a design.

These older devices all used the same thermal values for matching packages, which was not very accurate. We have started integrating data from multiple sources to improve the thermals, but not all devices have been updated yet.

I looked up similar devices in our portfolio that have had updated thermal models applied, and it looks like the θJA of 64°C/W is a bit on the low side. I would expect it to increase some, but not much, when we apply our newest thermal models to it.

I looked at 16 similar HC family NS package devices that have been updated, and I saw that the θJA value ranged from 78°C/W to 95°C/W, with the majority of them being very close to 87°C/W.

• Hi Maier

Thanks for the detailed explanation.

Now customer understood new θJA for HC family NS package will be close to 87℃/W.

It seems TI's measurement is based on JESD51-2A, which tested with 70um thickness pattern.

But customer uses 36um or 18um thickness pattern so customer is thinking the θJA will be worse than which shown in datasheet.

So customer wants to know do TI have θJA measurement data(e.g., xx% worse or no change ) for 36um or 18um thickness pattern.

Could you help confirm with it? Thanks.

Thanks.

Regards,

Jo

• Hi Jo,

I can request custom thermal models to be created, however I don't have any custom data for our logic parts.

Is the customer really planning to push a logic gate to its thermal limits?