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LMR36015: How does the capacitor between the pin of Boot and SW charge up during chip start-up time?

Mark-Lee
Prodigy 60 points
Part Number: LMR36015

Hi TI expert，

The function of the capacitor between BOOT and SW is providing voltage for internal high-side driver. In the start time, If the high-side tube is turned on first, how does it turn on when the capacitor is no voltage? But the low-side is turned on first, and high-side tube is turned off, so there is no way to charge the capacitor? I want to know how this capacitor is charged.

  • +1
    TI__Guru 71385 points

    Hi Mark,

    There is an internal diode (not shown) connecting VCC and BOOT pins.  When the low side FET is turned on, it will pull SW to GND, and the charging current will flow through the following path::

    VCC capacitor positive lead --> VCC pin --> internal boot diode --> BOOT pin --> Boot capacitor --> SW --> GND --> back to VCC capacitor Ground lead.

    In this way, the capacitor is charged. Hope this clarifies.

    Thanks,

    Youhao