TPS552882: Power management forum

Hi Boris,

Thanks for reaching out.

It looks like the stability problem regarding to the large ripple during heavy load. Please share the ESR value and part number of the output capacitors (C2907-C2910)? So we can check the compensation parameters.

BRs,

Bryce

Hi Bryce,

Thank you for your reply. The material part number is TCSOPL1A226M8R and the ESR value is 200m ohm.

The following datasheet is for your reference.

 TCSOPL1A226M8R.pdf

Thanks.

Hi Boris,

Sorry to miss the voltage level label of Vout waveform on heavy loading, it is still on PFM operation with the ' heavy loading' condition, so the voltage ripple is large. 

I have checked the compensation parameters, it looks good with the current COMP parameters.

Please add more loading on the output side, when the device enters CCM from PFM, the voltage ripple will be much smaller.

If you want much lower voltage ripple all all load conditions, you can change the resistor R2915 to 0ohm to select FPWM. And there also will be no noise. The drawback is the efficiency will be lower at light load compared with PFM.

BRs,

Bryce

Hi Bryce,

After observing the waveform, we noticed ringing on the ripple in the heavy load and are seeking a solution to address it in the power path.

Best,

Boris

Hi Boris,

To capture the output voltage ripple, it is recommended to use the ground ring to probe the voltage as below and use only one channel voltage probe to forbid interference from other channels.

BRs,

Bryce

Hi Bryce,

Thanks for your information. I have one more question.
After removing Rsense(R2904) and shorting it, the noise from the power supply components disappears, but we still require current detection functionality for protection against excessive currents. Do you have any suggestions regarding this issue?

• Install Rsense

• Remove Rsense and short it.

Thanks,

Boris.

Hi Boris,

Thanks for reaching out. Please expect a late reply due to public holiday. We will be back on 6th May.

BRs,

Bryce

 Hi Boris,

What is the loading do you use in the waveforms above? The CC mode or CR mode?

And have you tried to use the ground ring to probe the voltage waveform and use only one channel voltage probe?

BRs,

Bryce 

Hi Bryce,

Consistent results were obtained when measuring the waveform in CC mode using a single channel.

We would like to know why it will change the frequency and the ripple will decrease when we short it.

Do you think adding a Pi filter between ISP and ISN would help solve this problem(the noise of components)?

best,

Boris.

Hi Boris,

Do you test at the same loading condition? Because from the above load current waveform, it seems the output current is not the same value. I don't think shorting and un-shorting the output current sensing resistor have that big difference on output voltage ripple frequency at PFM.

I am not sure if the noise is introduced by the test of different voltage probe channels, please use only one voltage probe on Vout to see if the noise still exists.

BRs,

Bryce

Hi Bryce,

Using a single channel to measure the ripple yielded consistent results as previously noted.

While bypassing the current sense resistor resolves the issue, maintaining the current limit function necessitates its retention.

• With Current sense

• Without Current sense

Best,

Boris.

Hi Boris,

Thanks for feedback.

But I don't think shorting and un-shorting the output current sensing resistor could have that big difference on output voltage ripple frequency at PFM, can you also probe the inductor current waveform by inserting a wire between the inductor and the SW2 pad like below?

Do you test with the electronic load at CC mode? Because with different loading current, the voltage ripple frequency will vary much.

BRs,

Bryce

Hi Bryce,

The measured waveform is as follows, with our load being the device itself rather than generated by the electronic load machine.

• With Current sense

• With Current sense (zoom in)

• Without Current sense

• With Current sense (zoom in)

Best,

Boris.

Hi Boris,

Thanks for the waveforms.

From my understanding, it doesn't make sense the output voltage ripple and inductor current waveforms show that big difference, it is more likely the device is operating at different loading.

1. Could you try to change the output sense resistor to 5mohm? Will the output voltage ripple frequency also change?

2. And what about adding one ceramic capacitor (1uF or 0.1uF) at the ISN net like below?

3. Can you try with electronic load at CC load to see if any difference?

BRs,

Bryce

Hi Bryce,

After switching to CC mode, the test results remained unchanged.(see the waveform)

The addition of either 1uF or 0.1uF at ISN similar results.

Once we receive the current sense with a 5m ohm from the supplier, we will conduct further tests and share the results with you.

• Install current sense

• Remove current sense

Best,

Boris.

Hi Boris,

What about the output voltage ripple when the inductor current is continuous at heavy loading? For example 2A output current? Or change the resistor to R2915 to 0 to select FPWM?

You can use two 10mohm current sense resistor in parallel first to see if any change in test.

BRs,

Bryce

Hi Bryce,

When switching the Resnse to 5m ohm resistor, the resulting waveform exhibits a halved ripple size and doubled frequency.

However, the waveform appears to exhibit instability, raising questions about the reasonableness of this outcome.

• Install current sense(5m ohm)

Best,

Boris.

Hi Boris,

It is not the instability problem, it was due to PFM mode operation at light load. The only thing which confused me is the frequency of output voltage ripple varies with the Rsense value at PFM.

Would you select FPWM for smaller voltage ripple and constant output voltage ripple frequency? 

BRs,

Bryce

Hi Bryce,

After switching to PWM mode, the frequencies appear to be similar.

However, the reason for the frequency variation between PFM mode with and without Rsense installed remains unclear.

• Install current sense

• Remove current sense

Best,

Boris.

Hi Boris,

Below is the test result based on EVM. They show no difference on output voltage ripple frequency. So currently I think it is more related to the layout especially ISP-ISN and PGND-ANGD.

Output with Rsense: 9Vin-5Vo-0.5A

Output with Rsense shorted: 9Vin-5Vo-0.5A

BRs,

Bryce

Hi Bryce,

We separated the ISP and ISN pins and the result seems the same. We would like to know how to adjust the PFM frequency?

best,

Boris.

Hi Boris,

We have not seen an update from you for four weeks, so I assume the questions are answered and the issue is solved.

I close this thread now. If there is still something open, please reply and the thread will get opened again.

If you have any other question or of the thread has been locked, please open a new one.

Clicking the Resolved Button also helps us to maintain this forum.

Best regards,

Bryce