I'm putting together a circuit for a TI LM3549. It's an LED driver, so I assume there will be some amount of heat to manage, though the word heat doesn't appear anywhere in the datasheet, nor does the heat tab (or whatever the appropriate term for the slug on its underside is) have a corresponding pin number or name. Given this lack of information, I just left it floating. However, looking at the sample schematic on TI's evaluation board for this guy, I see a pin named that doesn't exist on the schematic - HTAB, and it's tied to ground:

enter image description here

I'm making the assumption that it's that slug, and that they're grounding it to link it to a larger copper area for heat dissipation. So, two questions:

  1. Am I interpreting that correctly?
  2. If so, how in the world can I make that conclusion from the datasheet?
  • 3
    \$\begingroup\$ Wow, that's a surprisingly big omission. \$\endgroup\$
    – pipe
    Dec 25 '17 at 22:28

In general, the thermal pad is grounded. Your interpretation of the missing information is correct.
Apparently, you are supposed to read some application note that is not referenced from the datasheet.

  • \$\begingroup\$ That app note does not really clear anything up either... "The thermal pad is usually tied to ground, and designers should verify the electrical correctness when connecting the copper planes to the thermal pad." \$\endgroup\$
    – Trevor_G
    Dec 25 '17 at 19:54
  • 6
    \$\begingroup\$ In general, the thermal pad is grounded, which is often enough enough to lull you into a false sense of security which bites you in the nethers when it isn't. Often split rail op-amps have a pad connected to V-, beware. \$\endgroup\$
    – Neil_UK
    Dec 25 '17 at 20:09
  • 4
    \$\begingroup\$ More general rule is that the thermal pad usually goes to the most negative voltage on the chip, but even that is not 100%. \$\endgroup\$ Dec 25 '17 at 21:10
  • 2
    \$\begingroup\$ Just to add a quirk.. this device has two grounds too... Murphy say's.... \$\endgroup\$
    – Trevor_G
    Dec 25 '17 at 21:52
  • \$\begingroup\$ Did you just happen to know about that app note? \$\endgroup\$
    – kolosy
    Dec 25 '17 at 21:56

Yes it is really unclear on the spec sheet and the app-note does not help either.

Without more information or calling the manufacturer you would need the part in hand to test it.

If you plan on going ahead before you get the parts, you really don't want to leave a big copper pad like that floating. Which is why it may be grounded on that schematic, but not shown on the part info, it may not be internally connected at all but just a thermal pad.

If in doubt it would be better to route it out through a small cap or resistor you can short out later when you have the parts in hand.

Adding the extra pin to the symbol will also make life a lot less confusing for the PCB designer later too.


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