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Original question:

LM25145: :

LM25145: RC snubber, gate resistors, and a Rbst in series with Cbst (C631).

Onur Kusakoglu
Intellectual 960 points
Part Number: LM25145
Other Parts Discussed in Thread: CSD18543Q3A, LM5146-Q1, LM61460

Good morning,

I used the quick calculator and webench tool and did my design.

Do you think should I add an RC snubber, gate resistors, and a Rbst in series with Cbst(Cbst is already in the design) by default or maybe not all but some of them? 

Input Voltage 24V Output Voltage 5.1V, 534 kHz Switching frequency, 47N SS capacitor(4ms), CSD18543Q3A is used for both mosfets (60V-35A). Continuous current 4.4A

I am really limited with the layout space at this moment but I also dont want to take a risk. If you suggest me to add any of the an RC snubber, gate resistors, and a Rbst in series with Cbst. 

If you need any other info please let me know.

Best Regards

Onur

  • 0
    TI__Guru 56622 points

    Hi Onur,

    Rbst, yes, but no need for the gate resistors or snubber if the layout is good. See app note snva803 for power stage layout guidance. Also, take a look at the LM5146-Q1 EVM for layout of a 8A/400kHz design.

    Note that we have a converter with integrated FETs for 24V to 5.1V/4.4A - see the LM61460 36V, 6A converter that is designed specifically for low EMI.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 960 points

    Good morning Timothy,

    Thanks a lot for the feedback. We tried to do the layout according to datasheet info. 

    We have no time for a re-spin of the layout so We would like to add gate resistors, Rbst and if it can fit to the layout also the snubbers.

    For gate driver resistors I am planning to add 0 ohm (402 package) resistors (Number 1 and 2 in the schematic) If necessary I will change it later.

    For Rbst resistor, like mentioned in the datasheet I would like to add 2.2 ohm resistor (402 package) (Number 3 in the schematic)

    For snubber resistor I will add an 1206 resistor 2.2 ohm and for capacitor 220 pF 50V 402 package. But I am not going to place them. If needed, I will measure the ringing frequency and adjust these numbers.

    What do you think about the connections on the picture above and the values and packages I have selected for passive components.

    Best Regards

    Onur

  • 0 in reply to Onur Kusakoglu
    TI__Guru 56622 points

    Hi Onur,

    Don't add a gate resistor for the low-side FET as it just affects deadtime performance and may lead to Cdv/dt shootthough. I wouldn't even include the high-side FET series gate resistor as all you really need is the Rbst resistor to control the high-side FET turn on.

    For the snubber, 0805 should be enough for the resistor and connect it to GND for better heatsinking and easier power loss measurement.

    More important is to optimize the layout - please see app note snva803 for power stage layout guidance.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 960 points

    Thanks Tim.

    Best regards

  • 0 in reply to Onur Kusakoglu
    TI__Guru 56622 points

    Onur,

    If there are no more questions, I will close out this thread.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Intellectual 960 points

    Hi Tim,

    I already marked your previous feedback. Sure you can close it. 

    thanks again.

    Onur