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LM5155-Q1: LM5155 blanking timer

David Daggett
Intellectual 945 points
Part Number: LM5155-Q1
Other Parts Discussed in Thread: LM3478, LM5155, LM3488

LM5155 boost controller. Does the 5155 have a blanking timer like the LM3478 & 3488?  That blanking timer is pretty important if using a flyback because of the snubber (always causes current spike).

  • 0
    TI__Mastermind 31125 points

    Hi David,

    There is no internal leading edge blanking circuit like the LM3478 & LM3488, but there is a blanking circuit of 50ns. This is seen in the minimum on-time specification in the EC table. If the 50ns is not enough, you can add a small external low-pass RC filter to overcome the current spike of the current sense signal.

    This is seen on page 22 of the datasheet:

    If required, a small external RC filter (RF, CF) at the CS pin can be added to overcome the large leading edge spike of the current sense signal. Select an RF value which is in the range of 10 Ω to 200 Ω and a CF value which is in the rage of 100 pF to 2 nF. Because of the effect of this RC filter, the peak current limit is not valid when the on-time is less than 2 × RF × CF. To fully discharge the CF during the off-time, the RC time constant should satisfy the following inequality:

    3 x RF X CF < (1-D)/Fsw

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Intellectual 945 points

    Thanks for the reply, Richard.  

    From our customer - I have already added the r/c to the boost circuit I am work on right now.  So the boost application is okay.  But the blanking time is really handy when designing a flyback because a flyback almost always needs a snubber capacitor.  Current from the capacitor can cause some pretty significant spikes across the sense resistor that could be difficult to filter out.  My question was more for other applications that would come up.  

    Any further guidance on that topology?

  • 0 in reply to David Daggett
    TI__Mastermind 31125 points

    Hi David,

    When designing a flyback converter, I would recommend utilizing the LM5155-1, which has the hiccup mode protection enabled. For designing the flyback converter, I would recommend looking at this application note: http://www.ti.com/lit/an/snva866/snva866.pdf which will cover the general design procedure.

    Thanks,

    Richard