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TPS56121EVM-601

Allan Cantle
Prodigy 20 points
Other Parts Discussed in Thread: TPS56121, TPS56121EVM-601

I am planning to use the TPS56121 DC/DC Converter in a new design and have acquired the TPS56121EVM-601 evaluation module. 

Unfortunately when monitoring the 1.2V output we noticed a 150mVpp ringing at approximately 200MHz every time the converter swtiched. This dampens down after about 10 cycles.

This noise is also more dramatically evident on the 12V input side though at over 1.5V in magnitude!

The Eval modules user guide has a graph that claims the output ripple noise should be around 20mVpp. However, on closer inspection, it does note that the bandwidth of the probe is at 20MHz! So what we are seeing could be correct??? 

Can TI support confirm or deny this feature that we have observed on the TPS56121EVM?

This will be un-usable for my application in the current state as I need a stable supply with less than +/-5% noise at 1.35V with switching load current changes of up to 10 Amps.

thanks

Allan

  • 0
    TI__Genius 17240 points

    Hi Allan,

    Is the noise observed at start-up or at steady-state? The high frequency noise may be coupled by the probing loop. Please make sure you follow the ripple measurement instruction described in the user's guide (Figure 3) and use 20-MHz bandwidth on the oscilliscope.

    If possible, could you provide the waveforms of these noise on Vout and Vin?

    Regards,

    Na

  • 0 in reply to Na Kong
    Prodigy 20 points

    Hi Na

    The noise is observed at steady state. 

    We are definately probing correctly. We've tried a x1 probe, x10 probe and differential probe and get all the same results. Also when we connect the pins of the diff probe together and probe the Vout we get a clean DC signal, so have proven that there is no misreading of the scope. 

    If we used a 20MHz setting on the scope we would not be able to measure the noise as it would filter it away, so that would be a little pointless. In fact that is my concern. i.e. that the PSU actually does radiate but the eval board user manual is not showing the bad news because of the 20MHz scope bandwidth.  

    Is the any chance you can look at the eval board at your end and turn the scope 20MHz bandwidth limit off. Either there is a fundamental problem with the stability of this chipset or the eval board isn't set up right. 

    Is this circuit available in the TINA simulator?

    thanks

    Allan

     

  • 0 in reply to Allan Cantle
    TI__Genius 17240 points

    Hi Allan,

    It's a common practice to measure output ripple with 20-MHz bandwidth limit. The reason behind that is the DC-DC controller is not designed to get rid of the switching noise but to compensate low frequency perturbations; there should always be some high frequency capacitors right at the load to bypass the high-frequency noise you have seen. There is nothing wrong with the stability of the chip or the EVM.

    There are transient model and average model of TPS56121 available in the TINA simulator. You can find it under the "Tools and Software" tab on TPS56121's webpage.

    Regards,

    Na