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D-CAP Regulator Control-Loop Bode Plot Measurement Setup

Danilo A.
Mastermind 34110 points

Hi Team,

We have received an inquiry from our customer regarding the application report entitled Measuring the Bode Plot of D-CAPTm, D-CAP2Tm, and D-CAP3Tm DC/DC Converters. According to our customer, 


As your documents shown, for external DCR Injection Circuit, we need to add some external components( Rp, Cp, and Cff). 

But for control-loop bode plot measurement setup, one 0.22uF capacitor is necessary. The design is given below.

We want to know if the design with 0.22uf capacitor loop test circuit will affect power performance (electrical parameters).And loop test circuit is only applied on verification phase or it can keep into official products. 

Regards,

Danilo

  • 0
    TI__Mastermind 18012 points

    Hi Danilo,

    We will look this then reply you soon.

  • 0 in reply to Miranda Gu
    TI__Mastermind 34110 points

    Hi Miranda,

    Thank you for your response. We will wait for your update.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Expert 5520 points

    Hi Danilo,

    The 0.22uF cap is only suggested to use when testing the bode plot. For the D-CAP converter with external RCC ripple injection circuit, it's recommended to use 0ohm instead of the 20ohm in the official products. And just pop 20ohm (or 10ohm/50ohm, depends on the requirement on your bode plot test equipment) replacing 0ohm when testing bode plot. 0.22uF is also used when testing bode plot.

    For the reason to use 0.22uF and its affects on the circuit, please see my explanation as below:

    1. Why need the 0.22uF cap? What will happen if not using it? For the D-CAP converter (not D-CAP2 or D-CAP3), it doesn't have internal ripple injection, so need the external RCC circuit to emulate the inductor current ripple (or it can be seen as ESR output voltage ripple). The voltage on cap Cp is just this ripple emulated voltage and injected into FB. But after adding a 20ohm resistor to test bode plot, the voltage on 20ohm resistor is also injected into FB. That makes the injected "ripple" value becomes larger. D-CAP control architecture applies COT strategy with valley voltage control, if the injected ripple voltage becomes larger, the average voltage will become larger with same valley voltage reference. So you can see the output voltage will become higher if just populating the 20ohm resistor to test bode plot.

    But if adding the 0.22uF cap parallel with 20ohm resistor, that makes the impedance of the parallel "0.22uF+20ohm" becomes much smaller at high frequency range. Since the injected ripple is with switching frequency at high frequency range, that will reduce the impacts of the 20ohm resistor for ripple injection and makes it more like a 0ohm resistor for the circuit operation.

    2. I think that's clear enough about the relation between "0ohm/20ohm/20ohm+0.22uF". 0ohm resistor is what we want for the external ripple injection circuit, but with 0ohm, we couldn't inject disturbance to test bode plot. So 20ohm resistor must be used to test bode plot. To avoid its effects on ripple injection circuit and converter operation, we add a 0.22uF parallel with 20ohm resistor to reduce the impedance at high frequency range to make it more like a 0ohm resistor.

    So I would recommend to use 0ohm in official products and pop 20ohm+0.22uF in bode plot test.

    Thank,

    Andrew

  • 0 in reply to Andrew Xiong
    TI__Mastermind 34110 points

    Hi Andrew,

    Here is the response of our customer,

    Thanks for your detailed explanation.
    These two HW design(with/without bode plot test circuit) is different,we don't want to update PCB version to avoid the cost,just change BOM,which is desirable?
    For official products,we don't pop 0.2uF cap and change 20ohm to 0ohm.
    For testing the bode plot,we pop 0.2uF cap and the injected res value is 20ohm.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Expert 5520 points

    Hi Danilo,

    The second one. Most customer use 0ohm resistor on their schematic and just change it when testing bode plot.

    Thanks,

    Andrew

  • 0 in reply to Andrew Xiong
    TI__Mastermind 34110 points

    Hi Andrew,

    Thank you very much for your answers to our customer's inquiries. 

    Regards,

    Danilo