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TPS54340B-Q1: Jitter in switching frequency of TPS54340B-Q1

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Jagdish

Replies: 11

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Part Number: TPS54340B-Q1

Hi TI support,

We are using TPS54340B-Q1 in one of application with design details of VIN=12V to 18V & Iout max=2.5A, Vout=4V, Fsw=917KHz. Input voltage is fixed at 15V. There appears jitter in switching frequency varying from 878KHz to 930KHz.

We tried varying resistive load from 0.74A to 1.6A but jitter remained persistent. Pls advice ahead for this issue.

Attached are the schematic snapshot & oscilloscope waveform of jitter.

Thanks,

jagdish

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Samuel Jaffe
- Apr 23, 2019 3:58 PM
Jagdish,

Please share the layout as well. I'd like to see if you could have some noise coupling to your RT pin.

-Sam

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Jagdish
- Apr 24, 2019 4:54 AM
In reply to Samuel Jaffe:

Hi Sam,

Please find attached layout.

Regards,

jagdish

LAYOUT OF iSLC3500-C REV.3.pdf
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Jagdish
- Apr 24, 2019 6:30 AM
In reply to Jagdish:

Hi Sam,

Apart from above issue, I would appreciate answers to below mentioned queries,

1. What could be impact on application with this jitter? As such we are fine with ripple we are getting. Any other impact..?

2. Could this impact reliability of TPS54340B-Q1 in long run?

3. Any other impact this jitter can have on any other parameter you think?

Regards,
jagdish
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Samuel Jaffe
- Apr 24, 2019 6:38 PM
In reply to Jagdish:

Jagdish,

1. Other than ripple you may run into issues if the frequency jitter falls into an EMI band you're not expecting. But at the same time the jitter will act as spread spectrum which may help EMI.

2. I have no evidence to suggest this will impact reliability of TPS54340-Q1 in the long run.

3. None that come to mind.

-Sam

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Jagdish
- Apr 25, 2019 5:06 AM
In reply to Samuel Jaffe:

Hi Sam,

Thanks for your response. Did you got the chance to look into the layout for any noise issue?

Your statement " I have no evidence to suggest this will impact reliability of TPS54340-Q1 in the long run" , do you mean here that this jitter can't have any long term reliability impact on IC. Do I understand correct? or you mean something other way, please clarify.

Regards,
jagdish
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Samuel Jaffe
- Apr 25, 2019 4:37 PM
In reply to Jagdish:

Jagdish,

The layout looks good as far as possible jitter is concerned.

I can't think of a way this jitter will impact long term reliability. You understand correctly.

-Sam

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Jagdish
- Apr 26, 2019 4:46 AM
In reply to Samuel Jaffe:

Hi Sam,

Thanks for response, it will help me at least to release this PCB version since IC has no long term reliability impact.

However, apart from layout what else could be reason for this issue. Any idea?

Regards,

jagdish
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Jagdish
- Apr 29, 2019 5:06 AM
In reply to Jagdish:

Hi Sam,

We found the issue to be noise pickup from nearby SMPS. Now having 240pF capacitor on 107K RT resistor makes switching frequency stable. We checked following after adding capacitor on RT pin.

=> Power on waveforms at 1.6A load appears clean.
=> No jitter at switching node.
=> Transient response slightly degraded from 126mV to 148mV but quite enough for our application. Slew rate for transient was 25000A/s.

=> So do you see any issues in having 240pF in parallel with RT capacitor?
Awaiting your response.

Regards,
jagdish
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Jagdish
- Apr 29, 2019 5:10 AM
In reply to Jagdish:

Sorry,

Forgot to attach waveforms for your review.

Regards,

jagdish
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Samuel Jaffe
- Apr 29, 2019 5:30 PM
In reply to Jagdish:

Jagdish,

That's great news! Yes a 240pF cap across Rt will help reduce any noise picked up on the RT pin. This is the solution I recommend to fix this issue. I don't expect any issues from this implementation.

-Sam

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