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LM61460-Q1: LM61460-Q1

Edward Chang
Intellectual 1630 points
Part Number: LM61460-Q1
Other Parts Discussed in Thread: LM61460

I wonder know why LM61460 need two additional pins of Rboot and Cboot?

Why not just use Rboot and Cboot at the external circuit without these two additiontal pins?Is there any special meaning for these two pins?

Thanks a lot!

  • 0
    TI__Expert 6245 points

    Hell Edward,

    Most of our Buck regulators use an n-channel high side MOSFET. These FETs require a gate voltage above the switch node to operate. We provide a BOOT pin to bypass the high side gate driver’s supply. A capacitor is connected between the BOOT pin and the SW pin. This capacitor typically has a few volts applied by the IC while the high side device is off. When the high side FET is on, this capacitor is used to power the gate of the high side FET.

    With most parts, if rise time is too fast, often it can be slowed down by starving the high side driver using a resistor between the BOOT pin and the capacitor used to bypass the BOOT node; we typically call this capacitor CBOOT. If the value of this resistor is kept small, typically a few Ohms, this method is safe and effective though not very consistent. As more slowing is needed, a larger resistor is needed. Larger resistor value resistor will typically trigger the BOOT circuit’s UVLO or cause the FET to not be driven correctly.

    On the LM61460, we have added an RBOOT pin that allows the slew rate of the high side FET to be adjusted without interfering with other functions of the BOOT pin such as CBOOT charging and proper sensing of BOOT UVLO.

    Regards, Robert