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TPS62A01-Q1: Stability question

juan carlos garcia tebar
Prodigy 10 points
Part Number: TPS62A01-Q1
Other Parts Discussed in Thread: TPS62A01, TPS62810-Q1, TPS628502-Q1

Tool/software:

Hi

We are trying to see if it is possible to close the loop of the DCDC using a remote sense. Please see simplified figure 1 attached

The Rlosses + Rsense is in reality a complex impedance. Hence the whole feedback system will look like figure 2

The realistic model for the load will be like picture 3

We did AC analysis of our fb network compared to that of the ideal case ( ie without the Z(w) of picture 3). It seems like if Cc>100pF, the effect of Z(w) is negligible since it gets bypassed by the Cap. See picture 4

However, I downloaded the PSPICE model of the DCDC from the TI website and ran transient simulation where after the DCDC is up I applied load step currents ( by closing switches with 2Ω loads). As can be seen in figure 5, for the second step the the DCDC has a strange behavior. I have tried tightening simulation tolerances etc but this behavior happens sometimes. Doing a sweep of CC it happens for some cases but not for others etc (ie might happen at 4nF but not at 1nF etc).

Can you help telling us how to proceed? we would really like to close the loop after the looses since otherwise we have excessive IR drop at the pin of the chip

  • 0
    TI__Genius 16940 points

    Hello Juan,

    TPS62A01-Q1 is not meant for remote sensing as it does not have VOS or dedicated AGND pin. That is why it is not possible to use differential feedback voltage sensing.

    One option will be to use something like below: capacitor divider helps to reduce noise and stabilize the senses voltage while resistor divider to scale the volatge.

    My second question is you write Cc=200pF and 1nF. What is that? Is it feedforward capacitor?

    Also which model you are using? Can you share the source please?

    Thanks,

    Farheen

  • 0 in reply to Farheen
    Prodigy 10 points

    Hi Farheen

    Picture below is what we are trying to do with Cc highlighted

    2. Model used was downloaded for here (under deign tools & simlulations):

    https://www.ti.com/product/TPS62A01?keyMatch=TPS62A01&tisearch=universal_search&usecase=GPN-ALT

  • 0 in reply to juan carlos garcia tebar
    Prodigy 10 points

    also we are just trying to compensate for the IR drops on the supply, not supply and gnd since as you mention this part doesn't offer differential remote sensing

  • 0 in reply to juan carlos garcia tebar
    TI__Genius 16940 points

    Hello Juan, 

    What is the device you are using: It it TPS62A01-Q1 or TPS62A01?

    Depending on that there is limit on feedforward capacitor. For TPS62A01, 120pF is the maximum allowed value. 

    Thanks,

    Farheen

  • 0 in reply to Farheen
    Prodigy 10 points

    Hi Farheen

    using TPS62A01-Q1 but there is no available spice model in the website hence I used TPS62A01.

    1.What is the Feedforward capacitor limit for the TPS62A01-Q1?

    2. Can you provide spice model for TPS62A01-Q1?

    3. Can you advice about stability of TPS62A01-Q1 given the parasitic network drawn in picture above?

    Thanks

    Juan Carlos

  • 0 in reply to juan carlos garcia tebar
    TI__Genius 16940 points

    Hello Juan,

    1) Recommended CFF is mentioned in D/S.

    2) It is in release process. So soon it should be in ti.com.

    3) How you added the parasitic inductance looks good. But the values of course you have to estimate depending how long these traces are.

    Also this model not designed for this kind of simulation so we do not know that it is the real behavior.

    My recommendation will be to use circuit like I have shared above: which will helps for stability and avoiding noise.

     

    But again we do not gauarantee the behvaior as this device is not designed for remore sensing.

    Thanks,

    Farheen

  • 0 in reply to Farheen
    Prodigy 10 points

    1.That is recommended but is there any maximum for CFF?

    2. The parasitic values are the actual extracted ones from our traces

    3. Can you advide on R & C values?

  • 0 in reply to juan carlos garcia tebar
    TI__Genius 16940 points

    Hello ,

    We do not recommend using values above the mentioned values in D/S as device is validated only with that. It can affect the stability performance.

    R values, you can choose according to resister divider equation: Vout=(1+R1/R2)x0.6

    Capacitors values should be in pF range. As a start you can run simulations from 100pF to 1nF and C2 10pF to 100pF. You need to run the stability simulation also to check that loop is stable and this device does not have average model for AC response simulations.

    I will recommend device like TPS62810-Q1 or TPS628502-Q1.

    Coming to first thing why Vout is dropping, have you simulated inductor current and switching frequency? Looks like device is starting again..

    Thanks,

    Farheen