SN6505B with two LDO outputs for 5V and 3.3V

Hello Mr. Rao,

I would like to use the SN6505b to output  two regulated voltages  with (LDO). : 5V and 3V3 and at least 500 mA (or more) at 5V.  The Input voltage at the Primary side should be 5V. I have checked the datasheet and it suggests the tranformer  "750315371"  for 5V to 5V applications but without LDO. for the 5V Output. I would really like to use LDO for the  5V since the voltage should be really stable. Do you have a siggestion for another transformer? This one has got the turns Ratio 1 : 1.1  I think I Need at least 1.4 or 1.5. 

What about the 3.3V ? is there any issue when connecting the second LDO for 3V3 at the Output of the 5V LDO one?   

If using the unregulated 5V Output  as suggested by the datasheet, could you please tell me how stabble it is?  Is it suitable to supplying  any usb Port for external USB Memory for example?

As General question : What is the Minimum Input current i should supply at 5V to the SN6505B Input  to get about 500mA.  Is the  switching frequency after powerering up the device always constant? Does it always Switch 1A all the time all the time

Thank you in advance.


  • Hello Mr. Gilbert,

    Yes, it is okay to have two level LDOs for generating two different output voltages with different current requirements. Since 3.3V is derived out of 5V output itself the overall output current would be reflected at 5V isolated output which shouldn't exceed 1A with input voltage between 3.3V and 5V.

    Transformer turns-ratio depends on quite a few parameters including LDO drop-out voltage. For detailed design procedure to design SN6505B circuit component values, please refer to Section 9.2 in SN6505B datasheet. Section in datasheet specifically talks about calculating turns ratio for your application using Equation (10). Assuming the below parameters,

    VFmax = 0.3V
    VDOmax = 0.3V
    VOmax = 5.1V
    VINmin = 4.9V
    RDSmax = 0.25Ω
    IDmax = IOUTmax/Eff = 600/0.85 = 705mA

    nmin = ~1.25

    For the above calculations turns ratio of minimum 1.25 would be required, please note that I have considered minimum input and output variations. For your application please choose these values as per your application requirement. Any transformer that meets the requirements of turns ratio, VxT product and isolation spec should be okay to use with the application.

    Regarding your question about unregulated output, SN6505B is an open-loop push-pull converter and hence output regulation largely depends on input voltage variation. If input variation is about 5%, output voltage would also have 5% variation assuming the tolerances of all other components (like transformer turns-ratio, RDSon, etc.) being 0%. I would recommend using LDO to get tight regulation at output.

    The average input current would depend on the load connected. For output load of 500mA, the typical efficiency of converter is about 85% hence average input current would be 590mA.
    When there is no external clock applied, the typical switching frequency of converter will be constant. Tolerance of switching frequency with variation in temperature is given in Electrical Characteristics of datasheet.
    Input switching current will also depend on the load connected, it will not be 1A always.

    I hope this post answers all of your questions, please go through design steps in Section 9.2 of datasheet for more details. Thank you.

    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Hello Mr. Rao,

    Thank you for replying and the detailed Information.  Regarding the turns-Ratio I also got the same values before  (Nmin = 1.25). But in the datasheet on page 3, you only recommend the "750315371 " for 5 to 5V applications.  In the example for the calculation of the V-T product of the SN6505B, you used different switching frequencies for both Input voltages (3V3 250kHz  and 5V : 300kHz ).  I can just find in the datasheet one minimum Switching frequency (363kHz). I used it to calculate my V.T Product at 5V iput voltage  5.5V / (2*363kHz) = 7.5VµS.  Is it the right frequency?

    I have found the transformer "750316028" with turns-ratio 1.7 and 1A. So that I can get about 9V at the Output and use a LDO for 5V Output? What  do you think? .In the datasheet you only  recommend it for 3V3 to 3V3 applications.

    Sorry for the detailed questions but I need more Information because we will not have any redesign for this product. So it Needs to work in the first turn.

    Thank you for your Support.



  • In reply to Gilbert Kuate:

    Hi Gilbert,

    Yes please go ahead and use 363kHz for calculating the VT product  for your application.
    The examples given in the datasheet are mock calculations and are not there to show default clock frequency of SN6505.

    Could you please share you email ID? I can share additional details over email.

    Thanks & Regards,
    Anand Reghunathan

  • In reply to Anand Reghunathan:

    Hello Anand,

    Thank you for replying. Here is my email address :

    Regardig the frequency "363khz" used for the calculation of the VT product. Is it in this case for 5V or 3V3? or  both? I used it for 5 to 5V Regulation.

    Please also let me know about other questions I asked before regarding the chosen transformer in order to have a 5V LDO regulated power supply with at least 500mA.

    Thanks in advance



  • In reply to Anand Reghunathan:

    Hello  Anand,

    After checking the different transformer offered  by Würth Elektronik on his Website (see Picture above) they have already defined Input voltages for each one. But the one they recommand for my application (5V to 5V) does not require any LDO at Output. But on my Opinion it is important for me to have some 5V LDO. I have calculated the turns Ratio at least  1:1.3  if usind an LDO. the suitable one I wanted to Choose was : 750316028  with 1:1.7 turns Ratio. But they only recommand it with an Input voltage of 3V3. Is there any issue if using them with 5V Input?  is it suitable to my application or I should use 1:11 one without LDO?

    Thank you for your Support



  • In reply to Gilbert Kuate:

    This thread has been closed over email.

    Anand Reghunathan