TPS54340B: Jitter on switching node of TPS54340B during specific Vin range

Hello Koji

Thanks for showing using our products

To better understand issue, could you please share comp pin waveform with RT = 124 k and Vin = 21.6V? 

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Thank you

Regards

Onkar Bhakare

Hi Onkar,

Thank you for yore reply.

I tried to capture the comp pin waveform with RT = 124k and Vin = 21.6V.

Once I connect a probe on comp pin, jitter on SW node is gone as following picture.

<With probe on comp pin>   RT=124kohm, Vin = 21.6V

<Without probe on comp pin>  RT=124kohm, Vin = 21.6V

Actually comp pin on my board is a little far from IC. (distance between comp pin to Rcomp is about 10mm)

Any advice for comp pin layout and around pattern?

Hello Koji

Kindly share the layout image of your PCB. Additionally, try increasing Chf (6.8pF) cap on the comp node to 100pF and see if it helps

Thank you

Regards

Onkar Bhakare

Hi Onkar,

As I mentioned before, My board is still prototype. So there is no artwork for this device.

But I attached the current my board pic as below. Please check it.

Also I'll try to change Chf(6.8pF) cap's capacity.

What do you think the cause of this issue from current situation and your knowledge?

Regards,

Koji

Hello Koji

The compensation components are quite far from the IC. It is likely that noise it getting coupled in comp pin as causing the jitter issue. Let me know does it improve after adding 100pF cap.

Thank you

Regards

Onkar Bhakare

Hi Onkar,

Thank you for your quick reply.

I tried to vary capacity of Chf and the best capacity is around 22pF and the jitter issue improved.

On the other hand more than around 22pF, lower input voltage range is unstable.

Do you think this phenomenon is normal?

Hello Koji

user4292685 said:

On the other hand more than around 22pF, lower input voltage range is unstable.

What is the voltage threshold now?

Additionally, could you please fill - SLVC452 Calculation tool | TI.com

Thank you

Regards

Onkar Bhakare

Hi Onkar,

Thank you for your reply, but I can't find the voltage threshold.

How do I get it?

Anyway I filled my design specification in TPS54360 family CALC TOOL and CHF(CPOLE) sets 47pF.

Regards,

Koji

Hi Koji

user4292685 said:

Thank you for your reply, but I can't find the voltage threshold.

I was asking about, at what voltage you are seeing jitter with increased Chf. Additionally, could you please share full calculator?

Thank you

Regards

Onkar Bhakare

Hi Onkar,

Is the voltage threshold you said the input voltage?

If so, it's about 20V. (not specific value)

and here is full calculator result.

Please check it and let me know if you find anything.

TPS54360-361FAMILY_CALC_TOOL_REVE.zip 

Hi Koji

I would recommend you change compensation network to following values.

Rcomp = 16.5kOhm

Ccomp = 4.7nF

It would help if could increase Chf to 100 pF or higher. Additionally, try placing higher value of capacitor at input. 

Thank you

Regards

Onkar Bhakare

Hello Koji 

Any updates on this?

Thank you 

Regards

Onkar Bhakare

Hi Onkar,

Sorry for late reply.

Could you tell me why Ccomp = 4.7kohm? Is this mistypo?

Regards,

Koji

Hi Koji

I apologies for mistake. It is a Typo. I have corrected it check

Thank you

Regards

Onkar Bhakare

Hi Onkar,

Sorry for late reply.

I tried your suggested Rcomp/Ccomp value and it looks less jitter.

So I will set these values as default setting.

Could you provide me the board plot for each Rcomp/Ccomp combination?

1. Rcomp=56kohm, Ccomp=680pF

2. Rcomp=16.5kohm, Ccomp=4700pF

Regards,

Koji

Hello Koji

The second sheet of the QuickStart plots bode for the selected component. You can also use WEBENCH for this purpose.

Thank you

Regards

Onkar Bhakare