LM22670: LM22670

Hi,

1) Can you share your schematic so that we can review for any marginalities?

2) Are the devices that do not exhibit this behaviour still getting reasonably hot?

3) Can you share the SW node and VOUT waveforms of the devices that exhibit this behaviour, when they are working properly (before they shut down)? And during the intermittent switching behaviour?

Regards,

Niranjan

Dear TI Support
Apparently, when using the LM22670 with polarity inversion, if the thermal pad is connected to the overall signal GND, an overcurrent will flow.
The LM22670 datasheet says that the thermal pad should be connected to GND.
This seems to be different between Philippine and Malaysian products.
The Philippine version is fine if it is connected to GND, but the Malaysian version needs to be floating from GND or connected to a minus-5V power supply in order to work.
I'd like to know the difference.
The circuit is exactly the same as the datasheet.

Hello

I Understand that the thermal pad of the LM22670 is connected to the negative power supply in the case of polarity reversal, but there is no explanation of this in the data sheet. The data sheet's package dimensions state that the thermal pad should be connected to GND.

Isn't this misleading? The same data sheet contains a sample circuit for polarity reversal, but there is no explanation of the thermal pad at all. Isn't this likely to lead to mistakes when designing a polarity reversal circuit? I think the data sheet needs to be revised.

What I would like to know most at the moment is the difference between the Philippine-made and Malaysian-made products.

In the case of the Philippine-made product, it works even if the thermal pad is connected to the signal GND of the board, but the Malaysian-made product does not work. I need to explain to the customer the evidence of this.

Please investigate the difference in the connection between the thermal pad and the GND inside the device between the Philippine-made and Malaysian-made products as soon as possible.

Hello

Just to confirm, I think the Philippine-made devices are probably completely isolated from the thermal pad and the device's internal GND.

As proof of this, the Philippine-made devices worked properly even when the thermal pad was connected to the signal GND. However, starting with a certain batch, the devices were changed to those made in Malaysia, and the thermal pad was connected to the signal GND, which caused the devices to no longer work properly.

Even if the Malaysian-made devices appear to be isolated in standalone measurements, I suspect that when the thermal pad is connected to the system's signal GND, the resistance between the device's internal GND and the thermal pad shorts out the moment the power is turned on.

Please investigate this difference as soon as possible.

Hi,

1) Just to confirm, is this difference consistent across multiple devices from both lots?

2) Can you measure the resistance between thermal pad and GND pin of an IC from each lot using a multimeter?

3) Please share the top marking/image of an IC from each lot so that I can look into this further

Regards,

Niranjan

Hello

I've actually checked multiple units from both lots.

When I measured the thermal pad and GND pin before assembly, the Philippine-made unit was over 10MΩ, while the Malaysian-made unit was around 5MΩ. When I turned on the power on the Malaysian-made unit with the thermal pad connected to the system's GND, the resistance between the thermal pad and GND pin dropped to 3-4Ω.

As for the top markings, the Philippine-made unit has a printed 1-pin indication in bold letters. The Malaysian-made unit has a round indentation where 1 pin is located, and the letters are thinner.

Hello

The results of the test between the thermal pad and the GND pin were as follows:

There was no potential difference (no current flow) for the Philippine product.

There was a potential difference (current flow) for the Malaysian product.

It appears that if the thermal pad is connected to the system's GND with reversed polarity, the thermal pad and GND pin may or may not short-circuit depending on the country of origin.

What is your company's opinion on this?

Hi,

We are looking into this and will get back to you. Thank you for your patience.

Regards,

Niranjan

Hello

For the Malaysian-made parts, we decided to separate the thermal pad from the signal ground by cutting the PCB pattern, but the problem is with the Philippine-made parts, which have already been installed in our customer's systems.

Modifying the PCB pattern on those parts would require extensive work to remove them.

Therefore, I would like your opinion that even if the Philippine-made thermal pad remains connected to the signal ground, it will not affect future operation.

Hi,

I have contacted the appropriate team and we are looking into what is causing this difference. I can only comment once we have information on that.

Let me get back to you once I hear from them.

Thanks,

Niranjan

Hello

Thank you for your response. Our customer's delivery deadline is approaching, so I apologize for the inconvenience,

but I would appreciate it if you could investigate this as soon as possible. Currently, our customer's opinion is that the Philippine-made part is working, but they are concerned that if the thermal pad is connected to the system's GND, the LM22670 will be damaged over time, and they are rushing to reach a conclusion.

In the case of the Philippine-made part with reversed polarity, I would like proof that it will not be damaged even if the thermal pad is connected to the system's GND.

Hi,

According to the data from AT sites, the thermal pad should be internally connected to GND pin. So, the Malaysian made (where low impedance is present between pad and GND) is the expected one.

For the Philipines made one, if the connection is not proper it can cause some issues with heat dissipation. Are you observing any high temperature rise with that one?

Also, do you have a picture of the labelling on the reel which these devices came in so that we can trace back the exact lot.

Thanks and regards,

Niranjan

Hello

I understand that the Malaysian-made version is suitable.

We are currently working on revising the board. However, our customer has asked us to determine what to do about the equipment we have already shipped from the Philippines.

I must respond by today with a decision on whether or not the Philippine-made equipment also requires disconnecting the thermal pad from GND. In this circuit, only about 70 mA of current flows at -5V, so floating the Philippine-made thermal pad will not cause any heat generation problems (it only feels slightly warm to the touch).

If the Philippine-made thermal pad is not connected to the GND pin due to a fault, I would like your opinion on whether it would be acceptable to connect the thermal pad to GND.

I no longer have the label on the reel, but I am sending you the results of a terminal-to-terminal measurement between the Philippine-made and Malaysian-made parts.

The lot number of the affected device is written on this document.

Please reply as soon as possible.

LM22670_IV check_ePAD-GND_073025.pdf

Hi,

From an application standpoint, for the Philippine made ones, there should not be any risk in leaving the thermal pad connected to system GND as it seems to be isolated from the device GND. Also as the load is light thermals should not be an issue.

However, I would still recommend verifying the functionality of both the Malaysian one where thermal pad is cut from system GND and Philippines one where it is connected. Please monitor the temperature also to make sure it does not get too hot in either case.

For new boards, connecting the thermal pad to the device GND (negative rail for this) is the correct way.

Regards,

Niranjan

Hello Thank you for contacting us.
We understand that for the Philippine-made units, even if the polarity is reversed, there is no problem if the thermal pad is connected to the system's GND.

For the Malaysian-made units that already have components installed, we are currently working on modifying them to disconnect the thermal pad from the system's GND.

We plan to modify the boards for subsequent products.

Thank you.