LMP7704-SP: What is the thermal pad on the bottom of the package connected to electrically?

1. Neither. This device has no ground pin. Note 5 says that the pad is connected to the lid, and table 5-1 says that the lid is connected to V⁻.

2. No. If you connect the pad to any potential, it must be V⁻.

3. How much a via reduces thermal impedance depends on the properties of your board. Anyway, is there a noticeable cost to adding vias?

Hi Wes,

Here is my guidance on the LMP7704-SP thermal pad.

1.Is it connected to ground or is it floated?

The pad is thermally connected to the substrate, but electrically high-impedance and would be considered floating. The pad is electrically shorted to the lid, which is also floating. I am aware the current datasheet on ti.com says the lid is connected to V- but this is not correct, we are working to update this. The SMD and the package drawing you have shows correctly that the lid is floating.

2. Is it Ok to connect the thermal pad to ground to dissipate heat. Our application will be in space, so we will not have the luxury of any air flow.

Yes, this will be the best way to dissipate heat. Typically we will recommend the pad to be connected to V- or GND.

3. How many vias would you recommend to connect the thermal pad to the ground planes on the PCB? Due to the low 10mA maximum stated recommended operating current it appears that it may not be necessary to have a "lot" of vias. With the typical 0.2mm thermal vias that TI recommends with a 1mm spacing between vias it would be easy to fit 32 vias in the area of the thermal pad. Are 4 vias enough or should we add more to maximize heat dissipation? I did not design the footprint, but am reviewing it and using it and want to be sure that we have the best possible thermal solution. 

I believe you are referring to the ±10mA specification in the Absolute Maximum Ratings section of the datasheet. This spec should be interpreted as ±10mA flowing through the input pins in the case of an overvoltage fault at the input. See the claw curves and the output short-circuit current specifications that the output stage can source/sink much more than 10mA.

The exact thermal requirements of your board will depend on your supply voltage, your load voltage/current, and the power factor. All of which contribute to the internal power dissipation of the amplifier. For the best possible thermal solution, I agree that you can increase the number of vias in your power pad. I don't have a specific number, but you can fill the entire power pad with vias using the suggested 1mm x 1mm grid. Ideally, these vias will be connected directly into the main ground plane on board which will help dissipate the heat.

Regards,

Zach