• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 13 subscribers
  • Views 441 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

SN6505B: External Clock Requirements

SN6505B: 4V output using flipped transformer

SeattleEE
Intellectual 480 points

Part Number: SN6505B

In the 6505B datasheet, there's a good collection of transformers. The Wurth 75031xxx series is particularly nice because of size.The '012 from that series is 1.1:1, and is recommended by TI to go from 5V -> 5V (no LDO) in table 3. 

I'd like to take that part treat the secondary as primary to get 5V in, and 4.1V out (post diodes) and then LDO that down to 3.7V at 50 mA or so.

The transformer specs are (1=primary, 2=secondary as spec'd): N1 = 1, R1 = 0.31 ohms, L1 = 475 uH, N2 = 1.1, R2 = 0.33 ohms. L2 is unspec'd, but can be calculated at 574 uH.

The input VT of this transformer when used normally (pins 1 and 3 driven by SN6505) is 11Vus.

If the transformer is flipped and the SN6505B drives pins 4 and 6 instead of 1 and 3, then the SN6505B will see an even higher VT product, is that right? I've built (same as Fig 34 in spec) this and am seeing the target 4.1V after diodes under load. 

Does anyone see any problems with this arrangement or the math above? Specifically, flipping the transformer and treating secondary as primary? Isolation isn't critical. Thanks very much

  • 0
    TI__Genius 17100 points

    Hi SeattleEE,

    I'm glad to know the transformers included on our datasheets are helpful. Although secondary side inductance can be calculated, the increase in V-t product is not guaranteed since saturation current for the transformer's secondary side is not specified.

    Using flipped transformers can be done, and from the information you've provided it seems tests over load are consistently okay, however we encourage confirmation from the vendor of whether their part will operate as expected in this setup.

    The V-t product required by SN6505B at 5Vin is fixed (~7.6Vus), so a higher V-t product supported by the transformer means margin for preventing transformer saturation. Which Schottky diodes are being used?

    Please reach out to Würth and let us know their response. From the SN6505B perspective, this is acceptable.


    Respectfully,
    Manuel Chavez