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LM5175: Different SoftStart depending on CSLOPE value

Shinichi Inoue
Guru 21626 points
Part Number: LM5175

Dear Specialists,

My customer is evaluating LM5175 and has a question.

I would be grateful if you could advise.

---

(1) SoftStart is not enough when CSLOPE is 33pF.

On the other hand, Softstart is normal when CSLOPE is 82pF.

I'd like to know the reason why the operation differs depending on the value of Cslope.

20181221_softstart of LM5175.pptx

LM5175 Schematic-1.pptx

ｰｰｰ

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    TI__Guru* 78100 points
    Hi Shinichi,

    Thank you for your interests of the LM5175. Actually the trend seems to be opposite, because smaller CSLOPE capacitor will create higher voltage at the PWM comparator during startup, which slows down the duty cycle growth and leader to longer soft start. The issue here seems to be related to a too small CSLOPE capacitor which produces too high slope signal which internally may be clipped, then the PWM comparator compares two flat top signal instead of one flat (COMP) and one ramp (CS + SLOPE), and the duty cycle can become large. If you compare 82pF and 150pF, you should see the SS with 150pF could be slightly slower at the beginning of the soft start.

    Thanks,
    Youhao
  • 0 in reply to Youhao Xi
    Guru 21626 points

    Hi Youhao,

    Thank you for your reply.

    I understand if CSLOPE is too small, duty cycle may become large.

    Could I confirm your comment.

    You mention 82pF and 150pF, which value is suitable of the customer's application.

    According to the excel calculation tool, it is no problem 150pF and 82pF.

    I appreciate your great help.

    Best regards,

    Shinichi

    LM5175 Vin19.2-28.8V Vout24V6.5A after xslx.xlsx

  • 0 in reply to Shinichi Inoue
    Guru 21626 points
    Hi Youhao,

    I couldn't attach xlsm file, I send a xlsx file.

    Shinichi