TPS62085 / Switching frequency variation

S.Satoshi

Other Parts Discussed in Thread: TPS62085, TPS62090, TPS62130

Hi Chris,

I found following thread that discussed about switching frequency of TPS62085.

Now I also found this behavior on EVM, but I can't find such behavior from information in datasheet. From fig.1 in datasheet, I assumed that it should be 2.4MHz at 3A load but it was approx. 1.4MHz with 3.3V input. On the other hand I can see 2.4MHz with 5.0V input. So please explain the mechanism of this switching frequency variation.

Please refer the waveform at 3.3V and 5.0V input I measured.

TPS62085EVM Switching Frequency_20151104.pdf

Best Regards,

Sonoki / Japan Disty

over 9 years ago

0 Chris Glaser over 9 years ago

TI__Guru*** 147250 points

There should be numerous threads on why the frequency of a DCS-Control device varies. It is not fixed in this topology. How is this important in your application?

0 S.Satoshi over 9 years ago in reply to Chris Glaser

Guru 21675 points

Hi Chris,
Customer expect fixed frequency at mid and heavy load. And there is no information such frequency varies. I think this note should be commented in datasheet.

Can you tell me the example thread that discuss about this behavior?

Best Regards,
Sonoki

0 Chris Glaser over 9 years ago in reply to S.Satoshi

TI__Guru*** 147250 points

Here is one such thread: e2e.ti.com/.../1570012

This question has been more common on TPS62090 and TPS62130.

0 S.Satoshi over 9 years ago in reply to Chris Glaser

Guru 21675 points

Hi Chris,

Your discussed minimum off time for TPS62130 and that is 130nsec.

e2e.ti.com/.../1373154

Can we use 130nsec as minimum off time for TPS62085 also?

Best Regards,
Sonoki

0 Chris Glaser over 9 years ago in reply to S.Satoshi

TI__Guru*** 147250 points

No, it will be different for different ICs. But the min toff affects high duty cycles. Most TPS62085 applications I see are low duty cycles.

0 S.Satoshi over 9 years ago in reply to Chris Glaser

Guru 21675 points

Hi Chris,

TPS62085 can be used with high duty cycle setting, so we should know the min toff information. Actually there is efficiency data with high duty cycle in Fig5 and 6 in datasheet. Can you ask designer about min toff?

Best Regards,

Sonoki

0 S.Satoshi over 9 years ago in reply to S.Satoshi

Guru 21675 points

Hi Chris,

I measured toff at Vin=3.3V, Vout=1.8V and Iout=3A. Then toff was 200nsec approx.. Is it expected operation?

Best Regards,
Sonoki

0 Chris Glaser over 9 years ago in reply to S.Satoshi

TI__Guru*** 147250 points

That is a normal operating point with moderate duty cycle. The min off time is found at high duty cycles. It is around 120 nsec.

How is the frequency important for your application?

0 S.Satoshi over 9 years ago in reply to Chris Glaser

Guru 21675 points

Hi Chris,

Thank you for your comment. There is no information of such frequency variation in datasheet. I need to know the behavior of this device as disty FAE. That is reason I'm asking this.

Best Regards,
Sonoki

0 S.Satoshi over 9 years ago in reply to S.Satoshi

Guru 21675 points

Hi Chris,

Please let me ask more about the operation under high duty cycle.
I understand that the Fsw will go down at high duty cycle condition, but then I'd like to know if there is possibility of unstable operation or jitter. Of course the external components value like inductor and capacitor follow the recommendation it datasheet.
Our customer is concerning about the possible of hearable noise from inductor due to jitter or unstable switching operation.
Please let me know the mechanism and foundation if there is no possibility of that concern.

Best Regards,
Sonoki

0 Chris Glaser over 9 years ago in reply to S.Satoshi

TI__Guru*** 147250 points

Since we began this conversation, we have released this app note: www.ti.com/.../slyt646.pdf

I've never heard of an audible noise issue at heavy loads. Any jitter would be well above the audio band.

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TPS62085 / Switching frequency variation