• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 210 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

TPS40170: why WEBENCH designer shows Qty=2 for M1 transistor, but on PCB layout is only one?

Krzysztof Nowaszewski
Prodigy 10 points
Part Number: TPS40170
Other Parts Discussed in Thread: CSD18531Q5A, , LM5145

Hello,

why WEBENCH designer shows Qty=2 for M1 transistor, but on PCB layout is only one? (check attached WEBENCH report)

This not happen with switching frequencies below preffered eg. 300 kHz.

26...42VDCx24VDC @10A 500kHz.pdf

Best regards

  • 0
    TI__Guru 54822 points

    Hi Krzystof,

    This is a low dropout application (26Vin to 24Vout), so the drop of the high-side FET is important. At that operating point, the high-side FET has both high conduction looses (given the high duty cycle) and high switching losses at 500kHz. I think you can use one FET, but it should be reasonably low Rdson. A 60V NexFET such as the CSD18543Q5A (6mOhm at Vgs = 7.5V) or CSD18531Q5A (3.75mOhm at Vgs = 7.5V) is fine.

    My recommendation is to use the LM5145 here as it has better performance than the TPS40170 (the LM5145 is the next gen controller to the TPS part). Please use the LM5145 quickstart calculator tool to simplify component selection.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 10 points

    Hello Timothy,

    thank You, I will also consider use the LM5145. So WEBENCH, displaying Qt = 2, suggests using two FETs connected in parallel or how should I interprate that?

  • +1 in reply to Krzysztof Nowaszewski
    TI__Guru 54822 points

    Krzysztof,

    WEBENCH is likely designing for a high ambient temperature, hence the use of two parallel FETs for heatspreading. When you enter the MOSFET parameters in the LM5145 quickstart file, just check that the MOSFET power dissipation is < 1.5W.

    Regards,

    Tim