TPS54917: oscillation in no load

Allan Fan

Part Number: TPS54917

Hi Expert,

When customer test tps54917, they see a ~40Khz oscillated frequency at Vout in no load. Also, they observe the oscillation frequency would change if C4 changed. Do you have any idea about it?

Ch1: Vsw

Ch2: Iind

Ch4: Vout

Thanks,

Allan

over 4 years ago

0 Dorian Brillet de Cande over 4 years ago

TI__Genius 14876 points

Hello Allan,

Mean frequency value I see in the screenshot you sent is 746.5kHz, where do you see 40kHz?

C4 is part of the compensation network, if its value is changed then behavior will be altered as well.

Thank you very much!

Regards,

Dorian

0 Kao Mick over 4 years ago in reply to Dorian Brillet de Cande

Intellectual 330 points

Hi Dorian

We see that the more obvious RIPPLE is 1 / dt: 38.16kHz as shown by the cursor.

Is this also caused by replacing C4?

0 Dorian Brillet de Cande over 4 years ago in reply to Kao Mick

TI__Genius 14876 points

Hello,

As compensation is modified when C4 is changed, I would expect that C4 has an influence on the phenomenon you are observing.

Also, unity gain frequency is approximately 40kHz which is one lead here. Crossover frequency will change depending of compensation settings (C4/C28/C1/R1).

What change do you observe exactly when you replace C4?

What value is C4? What frequency do you observe?

Thank you,

Regards,
Dorian

0 Kao Mick over 4 years ago in reply to Dorian Brillet de Cande

Intellectual 330 points

Hi Dorian

After re-measurement of the wave shape, we think that the cause of the ripple should not be the parameter value of the compensation circuit.

We re-measured the corresponding waveforms of the switch node and the inductor current and output voltage.

Figure 1 is no load, Figure 2 is light load 1A, and Figure 3 is Transient 0A-3.5A @ 2.5A/us 1kHz.

When the inductor current is greater than 0A, the 38kHz ripple will disappear.

This is related to the situation of the SW / NODE's hi / low side mosfet peak, is this the main reason for this phenomenon?

0 Dorian Brillet de Cande over 4 years ago in reply to Kao Mick

TI__Genius 14876 points

Hello,

What about the tweaking of C4? What did it give as a result?

It seems that it is influencing your system but does not explain why your are seeing this because this should not create a ripple that high.

You also can see on the current waveform at 0A that you have peaks which are same frequency as ripple you see on Vout.

I think we should focus on compensation component values for this system.

What did you use to get C1/R1/C28 and C4 values.

Thanks,

Dorian

0 Kao Mick over 4 years ago in reply to Dorian Brillet de Cande

Intellectual 330 points

HI Dorian

The following figure is the compensation parameters currently used.
The difference of C4 changes, as shown below

R52 = 13K
R1 = 20K
R54 = 680R
R16 = 80.6K
C1 = 33pF
C28 = 820pF
C4 = 1000pF

C4:470pF Transient 0.5A/us @1kHz

C4:1000pF Transient 0.5A/us @1kHz

0 Dorian Brillet de Cande over 4 years ago in reply to Kao Mick

TI__Genius 14876 points

Hello,

How did you calculated those values?

I use the following App Note to calculate the component values: http://www.ti.com/lit/an/slva104a/slva104a.pdf?&ts=1589184895417

It is a different part but the compensation method is the same.

Using these equations I find different values than the ones you are using which can explain the unintended behavior.

Thank you!

Regards,

Dorian

0 Dorian Brillet de Cande over 4 years ago in reply to Dorian Brillet de Cande

TI__Genius 14876 points

Hello,

Did you modify component value on the compensation using the App Note?

Were you able to see any change?

I found C28=3.8nF/R1= 4.493kOhm/ C1=109.6pF/ C4= 2.62nF/ R3=422Ohm.

Let me know if you have any other questions.

Thanks a lot!

Regards,
Dorian

0 Dorian Brillet de Cande over 4 years ago in reply to Dorian Brillet de Cande

TI__Genius 14876 points

Hello,

I haven't hear back from you.

Were you able to fix this issue by modifying component value on compensation ?

Is there any further questions you may have?

Please, let me know.

Thank you very much!

Regards,
Dorian

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TPS54917: oscillation in no load