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LMR16030: Rating voltage of BOOT capacitor of LMR16030

Geraldo Gomes
Intellectual 365 points
Part Number: LMR16030

Hi,

The datasheet in the design requirements (section 8.2.1 - Table 2) specifies a maximum input voltage of 60V,

but in the "Bootstrap capacitor selection" (section 8.2.2.7) specifies a 16V bootstrap capacitor. It's right?
My idea about gate charging of high side mosfet is that the Vgs voltage must be positive, so that the gate voltage

must be greater than source voltage, so the bootstrap circuit adds the output voltage plus the input voltage

to achive a higer voltage to charge the mosfet gate. So, the maximum voltage across the boot capacitor is at least

equal the input voltage.

So where is the error, in my idea of bootstrap circuit or in the datasheet?

Thanks in advance.

Geraldo Gomes.

  • 0
    TI__Guru**** 191445 points
    The BOOT voltage is relative to the switching node. When the high side switch is off, the SW node is clamped at one diode drop below GND. During this time the BOOT cap is charged up to about 5 or 6V. When the high side FET is on, the BOOT pin is at VBOOT + Vin approximately. So the BOOT cap rating is 2.5 to 3 x the BOOT to SW voltage.
  • 0
    TI__Mastermind 23845 points
    Hi Geraldo,

    The BOOT capacitor is located between BOOT and SW node and it is used as a gate drive for internal MOSFET. Maybe this datasheet will show you more of the internal working of the HS gate drive. See the block diagram on page 15 section 7.2
    www.ti.com/.../snvsa09d.pdf

    Also in that datasheet you can see on the abs max table between CBOOT to SW is 5.5V.

    As for LMR16030 you can find the ABS max for BOOT to GND = 71V and SW to GND =65V therefore you can deduct that BOOT to SW is 6V.
    Therefore 16V rated capacitor should be more than enough.

    Thanks
    -Arief