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SN6507-Q1: Accuracy improvement - schematic review (SN6507DGQR)

Oscar Liborio
Prodigy 370 points
Part Number: SN6507-Q1
Other Parts Discussed in Thread: SN6507, LM317

Dear Team,

 We would like to have a feedback on our power stage for the SN6507DGQR.

  • Coul you please estimate the accuracy of the output? (our current estimation is ca. 25%) Also could you provide some suggestion on how to improve the accuracy? 
  • Do you also have any other suggestion, feedback on our current solution?

Additional info: the application is a isolated current source of 250 mA. The first stage uses the SN6507DGQR to implement a push and pull voltage converter form 12V to 30V (first page). In the second page we use the LM317M-Q1 to provide a constant current of 250 mA.

Thank you in advance for your help!

Let me know if you need additional information.

Best Regards 

Oscar Liborio

PMP31319_revA.pdf

  • 0
    TI__Guru 54545 points

    Hi Oscar,

    Thank you for reaching out.

    I have reviewed the schematic of SN6507 and it looks good. Regarding the questions on the output voltage accuracy, since SN6507 is an open-loop push-pull transformer driver, the output is not going to be regulated. Since the output voltage is not regulated, there is no accuracy spec that we will be able to guarantee.

    The output voltage accuracy will depend on all the components that are in the path between input and output which includes input voltage accuracy, transformer tolerance, and the diode forward voltage variation. If you have all these information then you maybe able to estimate.

    Since you are using the regulator LM317 at the output of SN6507, the final output would be regulated and SN6507 regulation isn't necessary.

    I would also like to mention that SN6507 support an input current of upto 500mA. Since you have 12V input, the maximum input power supported would be 6W. Assuming an efficiency of 85%, the expected output power would be 5.1W. This means that the output current supported at 28V would be ~180mA max.

    Let me know if you have any questions, thanks.


    Regards,
    Koteshwar Rao

  • +1 in reply to Koteshwar Rao
    TI__Prodigy 370 points

    Hi Koteshwar,

    Thank you!

    I still have a question considering the maximum input current. In the datasheet it mentions in the recommended condition that the max switch current is 0.5 A.

    -Is this an average current? 

    -Also is there an internal function limiting this current or it is "just" a recommended condition.

    I would say it is "just" a recommended operating condition since in the absolute maximum ratings it says1.6 A as max peak current. I am asking this because the device will be oprated for around 300 ms  and then turned off, so it might still be able to provide the 250 mA  even at 28 V.

    Also where did you get the 85% efficiency? I know it changes based on the transformer but here it says that based on our load and Vin it should be above 90%

    Best regards

    Oscar Liborio

  • 0 in reply to Oscar Liborio
    TI__Guru 54545 points

    Hi Oscar,

    Thanks for your inputs.

    Oscar Liborio said:

    -Is this an average current? 

    -Also is there an internal function limiting this current or it is "just" a recommended condition.

    Yes, 0.5A is the maximum average current supported by the device and this not current limit but the device is characterized and qualified to support 0.5A average current across its life.

    Oscar Liborio said:
    I am asking this because the device will be oprated for around 300 ms  and then turned off, so it might still be able to provide the 250 mA  even at 28 V.

    Thanks for clarifying this. If I understand it correctly the device will be ON for 300ms every second and OFF the remaining time. This is only 30% of the time and hence, the average current consumed by load goes down. With this assumption, SN6507 should be able to support 250mA load with 30% ON or operation time.

    Oscar Liborio said:
    Also where did you get the 85% efficiency? I know it changes based on the transformer but here it says that based on our load and Vin it should be above 90%

    I didn't refer to any efficiency plots while making my statement in my previous response and assumed a worst-case efficiency of 85% for quick calculations as both 85% and 90% would still have led to the same conclusion with 100% ON or operation time.
    Your understanding is correct that SN6507 with Wurth transformer 750319692 yields over 90% efficiency at 250mA load.

    Let me know if you have any questions, thanks.


    Regards,
    Koteshwar Rao