INA826: Thermal Pad issue

Sabine Pennavayre-Guirand

Part Number: INA826

Hi,

When we draw our layout, we used the INA826 datasheet revision F, so without any specific information on how to connected the thermal pad, we connected it to the GND.

But it is powered by +/- 15V and we only discovered this week about the reivion G which states that the thermal pad should be connected to V-, so -15V in our case.

We have 4 IC on our board and we run our board for more than 72h without any issue with the thermal pad connected to GND.

As soon as we put our board in the temperature chamber, when the temperature reached 70 degC, the system current increased and the +15V was increasing until it reaches its protection voltage and turn off.

When we removed all the INA826, we had no issue. When we reconnected the INA826 with unconnected thermal pad, we had no issue. When we connected one of the INA826 thermal pad to the GND, we now had issue at ambient temperature. When we connected one of the INA826 thermal pad to the -15V, we had issue at ambient temperature.

Can you explain why we haven't seen any issue at ambient temperature and we only saw an issue at high temperature?

What are the risks to left the pad unconnected?

Thanks

over 4 years ago

0 Marek Lis over 4 years ago

TI__Guru 67791 points

Sabine,

Almost all modern semiconductor products, including INA826, use p-substrate process to implement the transistor-level circuitry, which require the p-substrate to be biased at the most negative supply voltage used to power the IC. Thus, connecting the thermal pad (located at the back-side of the silicon wafer) to any voltage above (V-) may result in current leakage that increases at the elevated temperature – see below.

0 Sabine Pennavayre-Guirand over 4 years ago in reply to Marek Lis

Prodigy 20 points

Thank you for the explanation.

What is the risk if I left the pad unconnected as the datasheet recommend to connect to V- or leave floating?

0 Marek Lis over 4 years ago in reply to Sabine Pennavayre-Guirand

TI__Guru 67791 points

Since the p-substrate of INA826 is independently biased from the top of the wafer, it is acceptable to leave the thermal pad electrically unconnected - this is the same as leaving it floating. Just do not connect it to any voltage other than (V-). However, the thermal pad must be soldered to the printed circuit board for thermal and mechanical performance - this assures physical integrity of the package.

0 Sabine Pennavayre-Guirand over 4 years ago in reply to Marek Lis

Prodigy 20 points

What are the advantage and inconvenient of leaving the pad floating compared to connecting to V-?

0 Marek Lis over 4 years ago in reply to Sabine Pennavayre-Guirand

TI__Guru 67791 points

Electrically there is no difference between the two options since the substrate is internally biased at negative supply. But practically, in a single supply configuration soldering the thermal pad to the ground plane of PCB and biasing at GND potential provides superior mechnical and thermal performance.

0 Sabine Pennavayre-Guirand over 4 years ago in reply to Marek Lis

Prodigy 20 points

So in our configuration, powered by +/-15V, it is better to connect it to -15V?

0 Marek Lis over 4 years ago in reply to Sabine Pennavayre-Guirand

TI__Guru 67791 points

Sabine,

In your configuration with +/-15V supplies, you MUST either connect the theramal pad to -15V or leave it floating - there is no other choice. BUT whatever you decide to do, the thermal pad must ALSO be soldered to the printed circuit board for thermal and mechanical performance.

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INA826: Thermal Pad issue