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LM2841 Bootstrap Capacitor Voltage Rating

Other Parts Discussed in Thread: LM2841

When using the LM2841, I was wondering how high the voltage rating needs to be on the bootstrap capacitor, Cboot.

For example, if one were regulating from 24V in down to 3.3V out, then I would assume that when the switching node voltage goes from 24 volts to a diode drop below GND, then there is already a little above 24V across Cboot.

In the Absolute Maximum Ratings section of the datasheet it says, "CB Voltage above SW Voltage = 7V"

Does this mean that somehow the CB voltage above SW will not go more than 7V above the SW voltage?

In other words, does the capacitor need to be rated above 7V, above 24V, or something higher even?



  • Hello Brian,

    When the switch turns off and the switch node and one end of the cap is at ground it will be charged by an internal low voltage source around 6.8V. Then this capacitor supplies the switch driver with that voltage above the drain of the switch for the on cycle, so the capacitor just rides with 6.8V on top of the switch node voltage. It will never see more than about 7V, so a 10V capacitor is fine regardless of what the input or output voltages are.