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LM5009: Boot RC to slow down the switching edge

Neo.Wang
Intellectual 980 points
Part Number: LM5009


Hi TI experts,

We are debugging an issue and found LM5009 switching noise is the cause of the issue.

We tried to add 10ohm resistor in series with BST cap, and then that issue got improved/fixed.

so can we use 10ohm+0.022uF Boot RC values? Please confirm.

Thanks

  • 0
    TI__Mastermind 23845 points

    Hi Neo, 

    You may want to check this over your operating condition especially at the lowest VIN that you have on your system. 

    The value seems reasonable normally we put around 1 -  4.7 ohm to slow down the rise time of the switch. 

    Alternatively you can put a snubber RC circuit across the diode from SW to GND to slow down the rise and fall time of the switch node. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Intellectual 980 points

    Hi Arief,

    Thanks for reply. What's the main concern of adding the series resistor?

    My understanding is the higher switching loss of top FET inside the LM5009, thus high temperature rise of this part, any other impact?

    Why Lowest VIN affect most?

    For our application, the input is 52-56V (typical 54V), output is 3.3V, max current is less than 100mA.

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    Intellectual 980 points

    And also, if we go with snubber, what RC value should i start with?

  • 0 in reply to Neo.Wang
    TI__Mastermind 23845 points

    Hi Neo,

    The concern is that with adding series resistor too large, there will be too much delay in charging the boot capacitor due to RC time constant. The worst situation is at the minimum VIN close to VOUT or at high duty cycle condition where the off time is short. The boot capacitor is charged during the off time of the top FET. During off time, the diode conducts and the SW node is low, therefore the boot capacitor is charging at this point. 

    Since your minimum VIN is 52V and the VOUT is 3.3V, i think you may not run into this condition. 

    As for snubber network for the SW node, you can start with a value of 1 ohm and 1000pF capacitor

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Intellectual 980 points

    Thanks Arief. I will go with 4.7ohm Rboot +0.022uF cap, and also add the placeholder for RC snubber with the values you suggested. 

    Thanks for the help here. Please let me know if any further concern from your end.

    Neo

  • 0 in reply to Neo.Wang
    TI__Mastermind 23845 points

    No problem. 

    Thanks

    -Arief