LM25145: Noise observed and no output

Nanjunda M

Expert 2485 points

Part Number: LM25145

Hi,

We are working with the LM25145RGYR in one of our boards.

We are generating a 5V from 28V with this circuit. Below is the schematic of the same.

We have observed the output voltage is at 1.1V. and there is a light sound coming out from the IC.

We tried probing the SS pin with a multimeter and found that the voltage is 150mV.

Also the other pins RT-0.99V, Comp -330mV, 7.5VRegout -6.44V, Sync IN- 2.1V, SW-180mV, PG -0V.

Has anyone seen this behavior?

Can anyone please help me to move forward.

Thanks & Regards,

Nanjunda M

over 7 years ago

0 David Baba over 7 years ago

TI__Mastermind 40060 points

Hello Nanjunda,

The Design looks OK to me. Sounds like the part might be damaged, have you tied replacing and retrying? Also, it would be helpful to look at VCC Pin to confirm the VCC regulator is working? Also suggest looking at the Vswitch node, Inductor current, Vout and SS pin during Start up?

Hope this helps?

David.

0 Nanjunda M over 7 years ago in reply to David Baba

Expert 2485 points

Hi David,

Thanks for the inputs.
We have not tried replacing yet.!
VCC seems to be working as we see about 6.4V on this line as I had indicated in my post.

Nanjunda M

0 Timothy Hegarty over 7 years ago in reply to Nanjunda M

TI__Guru 74422 points

LM25145 Wide Vin Synchronous Buck Controller Quickstart Calculator r12-7 -- VOUT = 5V.xlsm

Hi Nanjunda,

Your compensation values are not providing enough phase margin and the design is likely unstable.

Please review the quickstart file included here (also available by download from the LM25145 product folder). I used an effective Cout for each cap of 40uF (total 320uF). Once the recommended compensation values are installed, one way to check loop performance is by reviewing the load transient response.

Regards,

Tim

0 Nanjunda M over 7 years ago in reply to Timothy Hegarty

Expert 2485 points

Hi Tim,

We had referred the Webench design since that seemed more relevant than this Calculator.

meantime we had referred the calculator and it was almost similar values for the requirement. As attached below:

LM25145 Wide Vin Synchronous Buck Controller Quickstart Calculator r12.xlsx

Please suggest what we can do with this? How can we proceed?

Thanks & Regards,

Nanjunda M

0 Nanjunda M over 7 years ago in reply to Nanjunda M

Expert 2485 points

Hi David/Tim,

We replaced the IC and we are observing a 4.24V at the output. Also we checked the FBpin and its showing 0.62V.

Here is the output snapshot.

And the Switching voltage is as below snap:

Can anyone please help with this? I have designed for 5V and getting only 4.2V which is not desirable for my board.

Thanks & Regards,
Nanjunda M

0 Timothy Hegarty over 7 years ago in reply to Nanjunda M

TI__Guru 74422 points

Hi Nanjunda,

The WEBENCH design seems to use electrolytic output caps with 14mOhm ESR, whereas your design is using 100uF 1210 ceramic caps (negligible ESR).

Also, when using the quickstart file, please ensure that the ceramic output capacitance derated for voltage is correctly entered (for example, a 10V rated capacitor at 5V is likely at 40-50% of its nominal capacitance value).

Furthermore, please use a decoupling cap from VIN to GND of the IC (this is not shown in your original schematic).

Regards,
Tim

0 Nanjunda M over 7 years ago in reply to Timothy Hegarty

Expert 2485 points

Hi Tim,

Thanks for the inputs.

We have updated the quickstart file, could you please review and let us know the comments.

LM25145 Wide Vin Synchronous Buck Controller Quickstart Calculator r12Updated.xlsx

With this set-up, we are able to get only 4.6V on board.

What may be wrong with this?

Thanks & Regards,

Nanjunda M

0 Nanjunda M over 7 years ago in reply to Nanjunda M

Expert 2485 points

Can anyone please help.

Thanks & Regards,

Nanjunda M

0 David Baba over 7 years ago in reply to Nanjunda M

TI__Mastermind 40060 points

Hello Nanjunda,

Is the output voltage always 4.6V, even unloaded? Or does the output voltage drop to 4.6V when you apply some load? Thanks.

David.

0 Nanjunda M over 7 years ago in reply to David Baba

Expert 2485 points

Hi David,

We have tried both configurations, with load and isolated all the load from the output of the LM25145. But the voltage level is still 4.6V.

Thanks & Regards,
Nanjunda M

0 David Baba over 7 years ago in reply to Nanjunda M

TI__Mastermind 40060 points

Can we take a look at the layout?

Thanks.

David.

0 Nanjunda M over 7 years ago in reply to David Baba

Expert 2485 points

Hi David,

It will be difficult to share the layout, though I can give you a screenshot of the Top layer.

https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/196/LM25145_5F00_Layout.7z

Thanks & Regards,
Nanjunda M

0 David Baba over 7 years ago in reply to Nanjunda M

TI__Mastermind 40060 points

Hello Nanjunda,

Can you unzip the file, or just pdf the layers as I am unable to open the attached file? Thank you in advance.

David.

0 Timothy Hegarty over 7 years ago in reply to Nanjunda M

TI__Guru 74422 points

Nanjunda,

Please forward the latest schematic plus the PCB layout for review.

Regards,
Tim

0 Nanjunda M over 7 years ago in reply to David Baba

Expert 2485 points

Hi David,

Please find the files below

Thanks & Regards,

Nanjunda M

0 David Baba over 7 years ago in reply to Nanjunda M

TI__Mastermind 40060 points

Hello Nanjunda,

I cant review your layout fully from the screen capture provided. However, I will say that the input caps in relation to the Top and Bottom FET are not ideal. I suggest closely following guidelines detailed in the Datasheet if you need to spin the board?

Irrespective of the layout, I re-reviewed this post and I would be interested to have another look at Vout (AC coupled) along with the inductor current? Also please can you take another scope capture of the High Side Gate drive and the low side gate drive waveforms, I am curious to know if the low side is inadvertently turning on (partially) when the high side FET is switching on causing partial cross conduction?

Hope this helps?

David.

0 Nanjunda M over 7 years ago in reply to David Baba

Expert 2485 points

Hi David,

Thanks for the inputs.

But, the issue was with input. The input capacitor as per the design tool excel sheet was just 4 x 10uF capacitors, and when we increased this to 16x 10uF, we are able to get the proper output voltage of about 4.92V. (Caps used are 10uF, 1210 package and with 50V DC rating)

Not sure why the calculator suggested the number of input capacitors less than the requirement.

Thanks & Regards,

Nanjunda M

0 David Baba over 7 years ago in reply to Nanjunda M

TI__Mastermind 40060 points

Hello Nanjunda,

The input capacitor calculation is dependent on two factors.

1. The RMS ripple current raiting
2. the allowable Delta V requirement.

The calculator gave a suggested value based on these two parameters.

Note that there are DC bias effects that reduce the effective capacitance which will increase the Delta V at the input.

Also Note (and its likely) that there maybe a input filter (Or parasitic inductance at input) interaction issue that may be resolved with with the additional placement of input caps?

Please see paper below detailing instability due to input filter.

www.ti.com/.../snva538.pdf

Hope this helps?

David.

0 Nanjunda M over 7 years ago in reply to David Baba

Expert 2485 points

Yes.
Thanks David.

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LM25145: Noise observed and no output