• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 63 subscribers
  • Views 239 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.

LM5155: Isolated Flyback questions.

Motoyasu Hattori
Genius 11015 points
Part Number: LM5155

Hi,

My customers has the following questions.

--

According to the datasheet P.32, Figure46. Secondary-Side Regulated Isolated Flyback, we can find PNP transistor. I understand the PNP transistor of the Optional Primary-Side Soft-Start circuit from the SS terminal is practically necessary because the COMP pin rises to the VCC voltage when the output is short-circuited. However, is there any problem with the rating of the COMP terminal without the PNP transistor? Please advise us.

Could you please advise us how to define the component value of RCOMP/COMP/CHF attached COMP pins? You can find RCOMP/COMP/CHF on Figure 33 of datasheet P.20.

--

Please advise us.

Thanks and best regards,
M.HATTORI.

  • 0
    TI__Guru* 78320 points

    Hi Hattori-san,

    Thank you for posting.  My impression is, if FB is grounded, then COMP pin can be pulled up to VCC.  However, if FB is not grounded, the internal COMP clamp circuit will be turned on and you should not pull up or pull-down the COMP with an external circuit, or damage can be caused.  However, let me double check with our design team regarding pulling up COMP to VCC, and get back to you next week.

    Regarding Figure 33, it is for boost converter, and you can refer to the following application note:

    https://www.ti.com/lit/pdf/snva824

    Thanks,

    Youhao Xi, Applications Engineer

  • 0 in reply to Youhao Xi
    TI__Guru* 78320 points

    Hi Hattori-san,

    I checked with our system engineer who defined the part, the COMP pin has a 2.5V clamp so you cannot directly pull up the COMP to VCC. A pull up resistor must be inserted between the COMP and VCC when you use that configuration, and the pull up resistor must limit the max current below 1.6mA.  Please refer to the datasheet EC table.

    Thanks,

    Youhao