LM3424: Where should TREF, TSENSE, TGAIN terminal be connected to when thermal foldback is not needed?

Kazuya Nakai59

Guru 21625 points

Part Number: LM3424

Hello guys,

One of our customers is evaluating LM3424 on their original PCB.

They don't need thermal foldback. So they left TGAIN and VS terminal open and connect TSENSE and TREF to GND.

Are these terminals setting not good for thermal foldback ignore?

Your reply would be much appreciated.

Best regards,

Kazuya Nakai.

over 7 years ago

0 Irwin Nederbragt over 7 years ago

TI__Mastermind 28655 points

Hello,

This is from a previous LM3424 thread: "If not using the foldback function leave Tgain open, tie Tsense to ground, and tie Tref to VS. VS should also have some minimal loading to ensure stability so connect a resistor of 100k from VS to ground."

Regards,

0 Kazuya Nakai59 over 7 years ago in reply to Irwin Nederbragt

Guru 21625 points

Hello Irwin,

Thank you for your reply.

Is the reason of the setting to set the ouput of subtraction amplifier connected to TREF and TSENSE to 1.24V?

Thank you again and best regards,
Kazuya Nakai.

0 Irwin Nederbragt over 7 years ago in reply to Kazuya Nakai59

TI__Mastermind 28655 points

Hello Kazuya,

That's not what the previous post said, it said Tref to VS, TSENSE to GND. Similar to an op-amp circuit, what happens when both inputs are the same? The previous post ensures the thermal foldback will not be engaged.

Regards,

0 Kazuya Nakai59 over 7 years ago in reply to Irwin Nederbragt

Guru 21625 points

Hello Irwin,

Thank you for your reply.

Could I ask you an additional question as the attached PDF file and please give me your comment after taking a look the attached file?

Thank you again and best regards,

Kazuya Nakai.

LM3424_Foldback_canceling060618.pdf

0 Irwin Nederbragt over 7 years ago in reply to Kazuya Nakai59

TI__Mastermind 28655 points

Hello,

It appears the previous post was incorrect, I apologize. I went too fast with adding this information. The NTC resistance drops with increased temperature meaning that input wants to be higher than the reference TREF. TSENSE should be tied to VS and TREF to GND.

Regards,

0 Kazuya Nakai59 over 7 years ago in reply to Irwin Nederbragt

Guru 21625 points

Hello Irwin.

I understood that the best pin connection for the temperature foldback canceling is the following.
1. TSENSE is connected to VS pin.
2. TREF is connected to GND.

Regarding No.1, you wrote in this thread that VS should also have some minimal loading to ensure stability so connect a resistor of 100k from VS to ground. In this case, is 100kohm needed?

Also our customer cancels the temperature foldback with TSENSE=TREF=GND. They want to use this connection because it is difficult to change PCB pattern for the connection as No.1 and 2.
I think the customer connection (TSENSE=TREF=GND) is not the best but it is not wrong to cancel the temperature foldback.
Do you think whether the customer connection has no problem or has problem?

Could you please give me your comment?

Best regards,
Kazuya Nakai.

0 Irwin Nederbragt over 7 years ago in reply to Kazuya Nakai59

TI__Mastermind 28655 points

Hello Kazuya,

I wouldn't recommend this especially if there is no Rgain resistor. if the gain of the circuit is low then it could work fine. As for the loading resistor, they can try it without if they don't have one on their board but if the thermal foldback circuit is being used there will be loading on VS.

Regards,

Irwin

0 Kazuya Nakai59 over 7 years ago in reply to Irwin Nederbragt

Guru 21625 points

Hello Irwin,

I'm sorry for my late response.

Let me confirm. The best way to cancel the temperature foldback is that TSENSE should be tied to VS and TREF to GND, isn't it?
Then, is a 100kohm extrnal resistor needed to connect between VS and GND to ensure the minimum current load?

Thank you and best regards,
Kazuya Nakai.

0 Irwin Nederbragt over 7 years ago in reply to Kazuya Nakai59

TI__Mastermind 28655 points

Hello Kazuya,

Yes, that is correct.

Regards,

0 Kazuya Nakai59 over 7 years ago in reply to Irwin Nederbragt

Guru 21625 points

Hello Irwin,

Is 100kohm resister needed? Or not needed?

Thank you and best regards,
Kazuya Nakai.

0 Irwin Nederbragt over 7 years ago in reply to Kazuya Nakai59

TI__Mastermind 28655 points

Hello Kazuya,

We recommend placing a 100 Kohm resistor on VS if thermal foldback is not being used. It may work without it but the phase margin on the VS reference may be low.

Regards,

Power management

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LM3424: Where should TREF, TSENSE, TGAIN terminal be connected to when thermal foldback is not needed?