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UCD9244+UCD74120 voltage changes dramatically

Expert 2985 points
Other Parts Discussed in Thread: UCD9244, UCD74120, UCD9224

Hi all,

I used UCD9244+UCD74120 to generate 1.0V to supply the Keystone II SOC's core power.

Now we have not mounted the SOC yet. We just debug the UCD9244+UCD74120.

And I met a problem that is the dramatically changing in voltage which UCD9224 detects.

My design is the same as page 40 in the attached pdf file 6403.K2H_K2EVM-HK_SCH_A104_Rev4_0.pdf EXCEPT I removed the  MCP9700AT so there is no temperature related voltage into the TEMP/AuxADC pin of UCD9244.

The TEMP/AuxADC pin of UCD9244 is just connected with one 1000pF capacitor to GND.

And in the Fusion Digital Power software, I got the voltage that the UCD9244 detects, namely is what the UCD74120 generates changes dramatically.

I must raise the Overvoltage value above 1.5V, otherwise this channel will be closed immediately.

Does the no connection to temperature related voltage to TEMP/AuxADC of UCD9244 causes this problem?

Or something else?

Regards,

Feng

  • Hi Feng,

    As your description, I think it is caused by loop instability. 

    Could you help to check your loop? I have some suggestion here: 

    [1] Make sure your crossover frequency is within 1/15 to 1/20 of the Switching frequency.

    [2] Set your Target Phase Margin on GUI  between 50 degree -60 degree.

    Best,

    Luke

  • Hi Luke,

    One thing I didn't mention above is that the feedback loop I design follows the pic below.

    I agree with your suspicion that there is something wrong in the feedback loop.

    But could you give me some hints to adjust the crossover frequency due to my poor understanding with DC-DC converter.

    Changing the inductor's value at the output stage? Or?

    Thanks.

    Regards,
    Feng

  • Hi Feng,

    So you are using TI reference design, that will be good. But we should be careful of the capacitor and inductor we use. For example, ESR of ceramic capacitor and Aluminum electrolytic capacitor will differ greatly, that will affect the Zero position of Power Stage.  Loop Compensation will be a separate topic, if you are interested in, we can discuss offline:  luke-lu@ti.com

    For this case, i will suggest to tune the loop using Fusion GUI.

    GO to Design window.

    Change the compensation mode to "Manual"  and modify the ZERO and POLE position until they meet your requirements.

    Best,

    Luke

  • Hi Luke,

    Thanks. I will contact with you through email and update the final results in this thread.

    Regards,
    Feng
  • After I changed the switching frequency to 799KHz, things goes well. This results in that crossover frequency is within 1/15 to 1/20 of the Switching frequency.

    Further optimization could be done to adjust the value of L and C but this need more background knowledge about DC/DC and its loop compensation.