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TPS53315: Mismatch in the datasheet calculated values and Webbench values

Shifali N
Expert 2175 points
Part Number: TPS53315
Other Parts Discussed in Thread: TPS53315, TPS548A20, TPS25982

Hi ,

I am using TPS53315 in my design. I sharing the webench simulation that I used to check my design here.

webench.ti.com/.../114

Vin=12V, Vout=5V, Iout=8A, fsw=500kHz, SS=2.8ms

1. As per the formula provided in datasheet for inductor calculation, i get L=2.2uH. But in webench simulation I don't find any 2.2uH option to select. Which inductor value should I consider

2. Also as per formula, if OCP is 15A the calculated RTRIP value as per datasheet should be 76.57kohm. But in webench only resistor values from 65.3kohm to 67.96kohm is present. Which value should i consider.

3. Also I have added extra output capacitors because I will be hotplugging SATA HDD device which will have 5V output of this regulator connected to it. In the earlier board designed with this regulator, because of the sudden inrush current the output voltage would drop to mV and then again get stable when HDD was connected.
Attaching the link for conversation regarding this issue with TI Engineer.

e2e.ti.com/.../3576582

4. Also I have attached my schematics for reference. There I have also added additional bulk capacitors near the SATA connectors to avoid any sudden inrush current issue. Will these additional capcitors cause instability issue.

5. Also can you please provide a proper ripple injection circuit value for my circuit so that I don't face any stability issue.

  • 0
    TI__Mastermind 24520 points

    The webench link goes to a TPS548A20 design with 4.4V output voltage and 10A output current. 

    1.  use 2.2uH

    2.  Use the datasheet equation result.

    3.    Where is the high side feedback resistor connected with respect to the additional caps next to connector.   

            What is the resistance of the pcb trace from the Cout to connector capacitors. 

              Was previous board modified and tested to a satifactory result.

    4.   Can you attach schematic again. 

  • 0 in reply to David_Daniels
    Expert 2175 points

    Hi David,

    With previous board we couldn't solve the issue completely. With some HDDs we were observing voltage fluctuation sometimes. Hence we are going for next revision of the board. 

    We are trying to provide maximum number of bulk capacitors to avoid any voltage fluctuation.

    1. While calculating N value, do I need to consider the capacitors that is placed near connectors.

    2. Currently considering 47uF*12, I have set the N value for 13.5. Kindly let me know whether its correct. fsw=500kHz. Or I further need to increase the N value.

    3. Whether selecting N>4 will have any adverse effect on stability.

    Attached schematics for reference. Kindly review the same.3660.Design14.pdf

  • 0 in reply to Shifali N
    TI__Mastermind 24520 points

    Change C318 from 330nF to 1uF

     

    1. Yes, consider the capacitors near the connectors.

    Additionally,  derate the capacitance for the dc bias.

                    Using the 47uF 6.3V ceramic, can be 10 to 20uF at 5V. 

    It will be depend on the component size.

     

    2 -3.  Greater than 4 should be stable.   At 13.5 , will have a lower crossover frequency and will be stable.

  • 0 in reply to David_Daniels
    Expert 2175 points

    Hi David,

    I have used ceramic 47UF 25V X5R 1206 capacitor, and 100UF 16V X5R 1210 which is near the connector.

     If i consider the capacitors near the connector as well, i get N=34.7 and only the capacitors near regulator i get N=13.5.

    1. Will greater N value cause any issue.

    2. Should i reduce the number of capacitors near the connector. During solving this hotplug issue we had observed that additional capacitors reduced this instability issue. Hence in next revision we are taking complete care so that we don't face any issue. But adding additional number of bulk capacitors should not create a new problem.

    3. Also should the inductor value be calculated for maximum Iout=12A or my load 8A.

  • 0 in reply to Shifali N
    TI__Expert 7500 points

    Hi,

    David is looking into this and will feedback to you soon.

    Thanks,

    Lishuang

  • 0 in reply to Lishuang Zhao
    TI__Genius 14455 points

    Hi Shifali

         Have you calculated the output capacitance at the connector based on the charge that needs to be supplied to the HDD? 

    Have you considered an eFuse - as this will control the output slew rate during a hot plug and ensure that voltage does not dip. Also, you might not need a lot of capacitance when using an eFuse.

    Regards,

    Gerold

  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    Expert 2175 points

    Hi Gerold,

    Thankyou for your response. 

    1. Can you suggest a efuse part number.

    2. I should be placing the efuse circuit between TPS53315 output and connectors

  • 0 in reply to Shifali N
    TI__Genius 14455 points

    Hi Shifali

        The TPS25982 seems like a good device that could work. Yes, I would expect the eFuse circuit to be placed between TPS53315 and the connectors. I have looped in the eFuse team to support you on this questions.

    https://www.ti.com/lit/ds/symlink/tps25982.pdf

    Regards,

    Gerold

  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    TI__Guru* 79181 points

    Hi Shifali,

    Thanks for showing interest in eFuse devices. Can you create a new thread and post your requirements there? 

    Regards

    Kunal Goel