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TPS546A24A: Start-up instability

Part Number: TPS546A24A

Hello E2E community,

I have done two power supply designs using TPS546A24A with pin strapping only. The PCB have been built and assembled and I'm experiencing some trouble on start-up due to instability.

-Input 10V, freq 550kHz, output 1.8V up to 6A

-Input 10V, freq 550kHz, output 1.1V up to 8A

As you can see on the picture below the chip is trying to start-up but is oscillating which is causing the chip to stop.

/cfs-file/__key/communityserver-discussions-components-files/196/0016.TIF

I have tried to play with different compensation setting using the excel spread sheet but with no success. I have read on another thread that it might comes from a too big inductor which can make difficult to the current loop to adjust my calculated value was around 1.7uH but I have used a bigger one to reduce a bit more the ripple.

Below are the two designs let me know your thoughts, is there something wrong ?

/cfs-file/__key/communityserver-discussions-components-files/196/0058370A_2D00_SCM.pdf

/cfs-file/__key/communityserver-discussions-components-files/196/0058371A_2D00_SCM.pdf

Cheers

Yann

  • Hi Yann

        I am assuming that you are starting up under no load. Can you please confirm?

    A good way to confirm if the design is stable or not is to run a a quick SIMPLIS or Pspice simulation. The models are available on ti.com for you to quickly check this in simulation.

    Regards,

    Gerold

  • Hello Gerold,

    Yes I forgot to mention it, the first try where with no load but I have connected an active load and did some tries at 0.5A, 1A, 1.5A with no success. 

    Since your message, I have managed to start it up at 3.5A, the problem is that I need it to be stable with light load too.

    I will have a go on Spice simulation.

    Thanks
    Regards

    Yann

  • Hi Yann,

    OK, please run Pspice simulation, let us know if you have further questions.

  • Hello Gerold and Miranda,

    I usually use LTspice rather thean Pspice, so I have tried to add the component into LTspice but for some raison it doesn't work (I have done it before).

    Therefor I have downloaded TI Pspice and run the start-up simulation, I can see instability. Then I played with the setting resistor in order to solve that problem but no success. So I decided to start from a working example and use the one the datasheet and simulate it with a 1.5A load still unstable. So maybe I have a setting wrong in Pspice but I'm not an expert yet. There are some obscure fault at the beginning of the simulation.

    I would normally spend more time on this but I have a tight schedule on that project so any input is good to take. I have attached the simulation project, if you can point what's wrong 

    /cfs-file/__key/communityserver-discussions-components-files/196/TPS546a24a-simu.zip   

    Thank you for your help
    Best regards

    Yann

  • Hi Yann,

    I took a look at your PSPICE schematic and was having trouble debug the errors. I had some convergence errors and such. In the interest of time, I ran some SIMPLIS simulations. Is this acceptable for the time being? I can revisit the PSPICE model, if that is important later.

    Here is the result with your design (particularly the compensation settings and the inductor selection):

    Here is the result after dropping the inductance to 1.2uH:

    Consistent with your results, I observe the startup failure and instability with the 2.2uH inductor and a successful startup with the 1.2uH inductor. Is it possible to try your application with a 1.2uH inductor potentially? I suspect this will help solve your issue. This modification will add inductor ripple current and may contribute to voltage ripple on the output if that is acceptable for your design. Alternatively, we can look at adding output capacitance and tuning the compensation if we need to further refine the design.

    Regards,

    Alec Biesterfeld

  • Hello Alec,

    Yes I had the same error but I don't know how to solve them. Thank you for the SIMPLIS simulation, I have ordered lower value inductance from what I read in another post, they should arrive today or tomorrow, I will make a try and let you know.

    Thanks 
    Yann 

  • Hello Alec,

    I have changed the inductor for a 1uH and increase the output capacitance by 47uF it brings it to a total of 127uF, it looks now stable across the full load range 0A to 5.5A.

    I'm a bit surprise because the calculated value is 1.6uH, it seems to be a quiet sensitive chip, some of the devices I have worked with more traditional were not that sensitive for a +-06uH variation. 
    I will now adjust the compensation loop with comp code 25 according to the excel spreadsheet.

    Thanks,
    Regards

    Yann

  • Hi Yann,

    This architecture is sensitive to inductance value. I would recommend reading section 7.3 of the datasheet for a more detailed explanation. 

    Please reach out if you have any further questions.

    Regards,

    Alec Biesterfeld

  • Hi Alec,

    Have any experiment been done trying to bring the chip down to -55°C? 

    Would it cause major failure of the chip or would it be just out of spec?
    Also would it be able to start at -55°C?

    Best regards
    Yann

  • Hi Yann,

    This device may be stored at -55C. However, its operation has not been qualified below an operating temperature of -40C. The datasheet specifies operation only till -40C. 

    Regards,

    Alec Biesterfeld