SN74AXC4T774: BQB (WQFN) package thermal tab

Ross Fleming

Part Number: SN74AXC4T774

The BQB package contains a thermal tab (unlike the other packages). The datasheet makes no mention of tying it to ground or any other level (only "The package thermal pad must be soldered to the printed circuit board for optimal thermal and mechanical performance."). The Ultra Librarian CAD ties it to ground. I have two questions: May the thermal tab be tied to ground? May the thermal tab be tied to anything other than ground?

over 2 years ago

0 Albert Xu1 over 2 years ago

TI__Mastermind 20545 points

Hi Ross,

Please refer to this FAQ: [FAQ] Where do I connect the thermal pad of the logic QFN devices?

0 Clemens Ladisch over 2 years ago

Guru 243090 points

Ground or floating; see [FAQ] Where do I connect the thermal pad of the logic QFN devices?

0 Ross Fleming over 2 years ago in reply to Albert Xu1

Prodigy 10 points

Unfortunately, that FAQ was created in 8/2018 from a post referencing the RGY package. This predates the part in question (7/2019). Has it been confirmed to remain accurate for all parts released since 8/2018? Thank you.

0 Albert Xu1 over 2 years ago in reply to Ross Fleming

TI__Mastermind 20545 points

Hi Ross,

That information is still accurate.

0 Clemens Ladisch over 2 years ago in reply to Ross Fleming

Guru 243090 points

The thermal pad is connected to the substrate, which is made of p-type silicon. It forms parasitic diodes with n-type silicon, i.e., with all transistors, diodes, and other components on the chip. To prevent these diodes from becoming active, the substrate must not be more positive than any other voltage, so if it is not floating, it must be connected to ground (or the negative supply on opamps).

0 Dylan Hubbard over 2 years ago in reply to Ross Fleming

TI__Mastermind 42600 points

Hey Ross,

To add to Clemens great explanation, that FAQ is still accurate for all Logic and Translation devices in QFN packaging.

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SN74AXC4T774: BQB (WQFN) package thermal tab