LM7480: LM7480 not starting up properly

Samuel Eriksson

Part Number: LM7480
Other Parts Discussed in Thread: LM74930, , LM74720-Q1

Tool/software:

We have a very similar issue to the following post but has a slightly different component:
https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1491709/lm7480-q1-lm7480-q1-not-operating-correctly?tisearch=e2e-sitesearch&keymatch=LM7480

This is the behavior we see:
At VIN = 24V, VS is about 2.5V. Q1 never connects. When triggering on rising edge, HGATE never gets high. I suspect LM7480 gets stuck in the startup process because it consumes so much current at startup that VS drops too low and LM7480 never pass this moment.
We have tried to follow the recommended application example "200-V Unsuppressed Load Dump Protection Application" in the datasheet Figure 10-20.

In the post above for a similar LM7480-Q1 issue, a TI representative linked to the information below regarding startup issues with LM74930. It seems like the TI representative suggested to include extra components which are present in the LM7493 application example:
https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1425702/faq-lm74930-q1-what-is-the-function-of-diode-on-vs-pin?tisearch=e2e-sitesearch&keymatch=LM7493

We find it strange that the recommended application example according to the datasheet is not working and that TI recommends a fix from a datasheet for another component. Is our schematic diagram correct and should Q1 connect at startup? If the schematic diagram is incorrect, what changes do you recommend?

1 month ago

0 Shiven Dhir 1 month ago

TI__Mastermind 47340 points

Hi Samuel,

Can you try the startup again with R73 lowered down to 4.7k?

Regards,

Shiven Dhir

0 Samuel Eriksson 1 month ago in reply to Shiven Dhir

Prodigy 20 points

Hi Shiven,

Thanks for the quick reply. I tested 5k (10k in parallel with 10k). VS is now 3.1V but Q1 is not turned on. I tried several power resets. The lab supply current limit was maxed out at 3A.
I also tried to increase the input voltage to 30V with no luck.

In one of the support cases above, the circuit worked after R73 was decreased to 1.6k. I am not sure if this was the final solution or if they found a better one.
Our implementation should survive 200V load dumps and according to the datasheet section 10.3.2.3 both the resistor and zener diode would have to be rated for much higher power with R73 1.6k. Please let us know if you have any recommendations.

In the datasheet section 10.3.2.1 about the boost circuit, a 100uH inductor L1 is mentioned. I can't find L1 in any schematic diagram in the datasheet.

Regards,
Samuel

0 Samuel Eriksson 1 month ago in reply to Samuel Eriksson

Prodigy 20 points

Hi again,

I found the error: R75 wasn't properly soldered to the GND pad which meant that the over voltage condition was triggered every time.
Now it also works with R73 = 10k.
In my previous post I made a comment about an L1 inductor mentioned in the datasheet. This question is still relevant.

Regards,
Samuel

0 Shiven Dhir 1 month ago in reply to Samuel Eriksson

TI__Mastermind 47340 points

Hi Samuel,

Glad you found the issue.

The section 10.3.2.1, I cant see any inductor. Are you looking at the right datasheet?

Regards,

Shiven Dhir

0 Samuel Eriksson 1 month ago in reply to Shiven Dhir

Prodigy 20 points

This is the component webpage I have used:
https://www.ti.com/lit/ds/symlink/lm7480.pdf?ts=1753219314817&ref_url=https%253A%252F%252F
It links to the following datasheet:
www.ti.com%252Fproduct%252FLM7480%252Fpart-details%252FLM74800MDRRR%253FkeyMatch%253DLM74800MDRRR%2526tisearch%253Duniversal_search%2526usecase%253DOPN

Section 10.3.2.1 on p27 is different than what you refer to:

Now I see that this section refers to another component LM7472x-Q1 which I don't understand.

0 Shiven Dhir 1 month ago in reply to Samuel Eriksson

TI__Mastermind 47340 points

Hi Samuel,

This is related to the controller LM74720-Q1 and you can ignore it. Inductors are not required to make this controller to work.

Regards,

Shiven Dhir

0 Samuel Eriksson 1 month ago in reply to Shiven Dhir

Prodigy 20 points

Hi, I was OOO. I think the LM7480 datasheet 10.3.2.1 section is misleading and should be edited. It could be a copy paste error from another datasheet.

0 Shiven Dhir 1 month ago in reply to Samuel Eriksson

TI__Mastermind 47340 points

Hi Samuel,

It indeed is a copy paste error from another datasheet. Thanks for the feedback.

Regards,

Shiven Dhir

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LM7480: LM7480 not starting up properly