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LM26420 / Phase margin and crossover frequency

ACGUY
Guru 11170 points
Other Parts Discussed in Thread: LM26420, LM26420XSQEVAL

Hi all,

Our Customer is considering 2-ch DC/DC from TI or Intersil.
I introduced to them LM26420.
However, the device is disadvantageous.
Customers want a phase margin of 60 degrees or more.
I attached simration result file for WEBENCH.

1 - Please let me know how to improve phase margin. (Target:more than 60 degree)
2 - When 2.5Voutput, crossover frequency is 17.487kHz. I think this responce is slow.
     Please let me know how to speed up the response of this device.

Regards,
Y.Nagai

LM26420X_5Vin.pdf

LM26420X_3.3Vin.pdf

  • 0
    TI__Intellectual 1775 points

    Hi Nagai-san,

    1- To improve the phase margin, You should add a feed-forward capacitor in parallel with Rfbt1 and another one in parallel with Rfbt2. Select the capacitors so that the "zero" frequency created by Rfbt and Cff is at the cross-over frequency. This should add a significant phase boost.

    2- Since the device has internal compensation, changing the cross-over frequency of the loop response cannot be achieved directly with compensation components. The addition of a feed-forward capacitor will have a small effect on that cross-over frequency but not much. That being said, you can increase the cross-over frequency by reducing the output capacitance. However this would likely increase the amplitude of the transient response on the output voltage. Do you know what the customer's requirements are in terms of load transient?

    Thank you,

    Florent Boico

  • 0 in reply to Florent Boico
    Guru 11170 points
    Hello Boico-san,

    Thank you for your response.

    1 - I will introduce related documents about Type-III compensation to customer.
    EN : www.ti.com/.../slva352a.pdf
    JP : www.tij.co.jp/.../jaja222.pdf

    2 - I could not get the detail for transient load information.
    Competitor's device ISL8033 is very strong.
    Phase margin : 80 deg,
    Crossover frequency : 50kHz
    However, if I get TI's predominance, I can drive out competitor from customer's board.
    If you have an AC-simuration tool of LM26420, could you provide it to me?

    Regards,
    Nagai
  • 0 in reply to ACGUY
    TI__Intellectual 1775 points

    Hello Nagai-san,

    I took an EVM and checked its 2.5Voutput (LM26420XSQEVAL). I got the following plot:

    The cross-over frequency is 36kHz with a 65degree phase margin.

    After adding a 180pF feed forward capacitor, we obtain the following measurements:

    The cross-over frequency is now 83kHz with a 93 degree phase margin.

    I took plots at 2A and also with 3.3Vin and they show very similar results (I could share that too if you want). So I think the performance of the device is quite good to begin with and can be easily improved by adding a feed-forward capacitor. That being said, the inductor on that board is 1uH which is 5 times smaller in inductance compared to the webench designs you shared with us. I could look at which limitation a 5uH inductor creates but I think you should also be able to reduce the inductance on your design without affecting the performances.

    Does that help?

    Best Regards,

    Florent