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LM5025AEVAL: Sync freq-Loop Feedback and Clamp cap

Irene Perez
Prodigy 150 points
Part Number: LM5025AEVAL
Other Parts Discussed in Thread: LM5025B, , LM5025A, LM5025

Hi, I´m evaluating the LM5025B Eval Board (3.3V@30Aout) due to the need of sync in a wide band of frequency. The result I´m getting is that for big frequency changes through the SYNC pin,there is a high undershoot at the 3.3V output at about 900mV in case of a change from 200kHz -> 400kHz. Is this behaviour the expected? seems that rigth after the freq change the duty increases and takes a while stabilize.

Any help on how to avoid such high overshoot?

I have another question related to the feedback loop of the Eval. After the type III compensator there is an additional filter:

 

What is the function of this second stage?

 

I´m also using the following data spread sheet to know the calculation of the different parameter of the eval board:

www.ti.com/.../sluc655  

  I see a discrepancy between the calculated value of the clamping capacitor and the final value of the PCB. It is 10 times the value calculated, why making bigger the Ccl? This limits the control loop bandwidth as the double pole formed by Ccl and LMag will be lower in frequency.

Waiting your update.

Many thanks,

  • 0
    TI__Mastermind 34465 points

    Hello ,

    The LM5025AEVAL can be synchronized to a SYNC frequency of 190kHz to 300kHz.
    The minimum SYNC pulse width is 100nS and the threshold voltage for the SYNC signal is 2V.
    Do you have any waveforms for your SYNC signal ?
    The User Guide specifies  an upper limit of  300kHz and you  are using  400kHz
    How does it behave at 300kHz ?

    The U3A opamp is set up as a unity gain high frequency filter with a roll off frequency of about 68kHz.
    I cant think of a reason as to why this was done except that the part is a dual opamp and the unused A packages can be configured this way and filter out any high frequency noise well above the overall loop bandwidth.
    Have you tried removing the C31 and checked the performance ?

    The document you referenced in your post is the LM5025A data sheet.
    I dont see an equation here for the active clamp capacitor .
    What is the Equation number or page that this is on ?

    Regards

    John

  • 0 in reply to John Griffin1
    Prodigy 150 points

    Hi Jhon,

    Thanks for your quick response.

    The LM5025A can be sync from 160kHz to 1MHz as per the info in the datasheet:

    Isn´t that correct?

    I dont have a photo of the SYNC signal, but I´m applying through a waveform generator a pulsed signal of the desired freq with a pulsed width greater that 100ns and an amplitud of 2V, this goes to the SYNC pin through a resistor to limit the current.

    The LM5025 is doing the sync in terms of frequency change pretty fast, but the output voltage response to this freq change is not appropiate for my application. Below there is a picture of an undershoot and overshoot when changing just the sync freq(output load is in steady state): 

    The over/under shoot is around 100mV, but when moving from 400kHz to 230kHz reaches 900mV:

    Regarding second point, I will try without C31.

    Regarding the clamp capacitor, the design calculator I´m talking about is in the following location:

    Waiting your update.

    Many thanks,

  • 0 in reply to Irene Perez
    TI__Mastermind 34465 points

    Hello Irene,

    I had not seen this note in the data sheet about there being  no constraint on upper limit to sync frequency.
    The evaluation board has a recommended upper limit of 300kHz.
    I think your tests demonstrate what happens when this upper limit is exceeded
    The parametric data in the data sheet do not specify an upper frequency limit :

    And the parametric data is what really matters in a design application.
    I think the data sheet should be modified to remove this section:

    . "There is no constraint on the maximum sync frequency." 

    or  it should explain in detail the constraints on using a very high sync frequency.
    However it is also reasonable to assume that a disturbance to the control system is going to cause a transient response in the output voltage.
    I recommend that output capacitance should be increased achieve the desired pulse response 

    Regards

    John