LM5104: About bootstrap capacitor

Hi Mayumi,

Thanks for your question on LM5104, Im an apps engineer supporting this device. The datasheet says that the bootstrap and VDD caps should be X7R or low ESR capacitors. Other types of capacitors may be used however X7R is what is recommended. Just be sure the cap is rated at least twice the VDD supply voltage. X7R caps are low ESR so the cap does not heat up too much or experience a large ripple during peak current draws in order to maintain a long lifetime and minimal derating when a DC bias is applied.

Electrolytic caps have a way of drying out and becoming less conductive so its important to consider the larger ESR giving way to more heat and lower lifetime. The larger ESR can also bring a larger voltage drop that can dip below HB UVLO. This can also hinder the gate driver peak sourcing capability for the highside because this larger ESR. Since electrolytics have inherently more leakage due to a weakness in dielectric we need to consider effects if a larger dutycyle is used to make sure the driver doesnt reach UVLO. This extra leakage current actually accounts for total charge supplied by the bootstrap and can attribute to more gate driver power loss since the bootstrap diode is internal.

Hope this helps your concerns, please let me know if you have any more questions.

Thanks,