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TPS56339: boot resistor vs EMI

Part Number: TPS56339


          My customer is using TPS56339 to power a RF module like product. They found increasing  the boot resistor between BOOT to bootstrap capacitor will improve the RF recevier sensitive. 

so far, they increase this res to 100Ω, the RF rev sensitive improve 7dbm(from -83 to -90). I can't tell why this change can get this result.   please suggest how big value this resistor can be used? and how doest this resistor

associate with device EMI peformance? thanks. 

  • Hi Yue,

    It is reasonable. The bootstrap resistor is used to slow down the turn-on slew rate of the high-side MOSFET. Slowing the high dv/dt transition on SW will result in lower ringing and result in less overall EMI noise. That's why the RF rev sensitive is improved.



  • Hi Yue,

    As Lucia said, the boot resistor will form a RC charging circuit with bst cap, to slow down the turn on of high side FET, reducing the ringing of SW. The larger resistor , the slower tuning on FET. But if this resistor is too large , it will make the bst cap can't get enough voltage to turn on the high side FET. it will lower eff , too. There is a trade -off for the EMI and eff performance . 

    Beside the bst resistor , a snubber circuit near SW pin will also help to reduce EMI lot.



  • Thanks.  I actually have already recommended them to use snubber circuit.  

    BTW, Do you know what may be the maxim value of this resistor to use? is there a recommendation? based on test or there is a formula here?

  • Hi Yue,

    Sorry that we don't have this value as it will vary with the VIN/VOUT/Loading and even layout will affect this data, too.

    Suggest to try it on EVM. If there is no many rings with a enough larger resistor and can keep IC work well , no need to enlarge it anymore .