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# Understanding the function of Boost capacitor

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#### FreshmanNewbie

##### Full Member level 6
I'm going through this MOSFET Driver IC - MIC5021.

Since this is a high side switch, the gate voltage should be higher than the source voltage, which would be the Source voltage, Vdd. According to the datasheet,

If the vdd voltage is 12V, then the gate voltage typ would be 18V, which seems logical. We might need to select the MOSFET's gate-source threshold voltage according to this.

One small question is what would be the gate voltage if the Vdd is 24V? What can be assumed so as to select the MOSFET gate - source threshold voltage?

But my actual question is how does the Boost capacitor help? Does the boost capacitor help to boost the Vdd voltage and provide the boost voltage to the GATE pin?

Also, different configurations are mentioned in Figures 6-1, 6-2 & 6-3. Do small capacitors imply small time constant, hence charging happens faster? But in that case, for high voltage configurations, the capacitor value should be less right? But for the highest voltage config of figure 6-3, 20V to 36V, the cap recommended is 10nF. But for 12V-36V configuration, the cap is lesser 2.7nF? Unable to understand this clearly. Request some help on this.

Hi,

If you do an internet search on "boost capacitor function" you get a bunch of good explanations.
This "boost" circuit is also called "bootstrap circuit". You may also do an internet search on this.

Klaus

MIC5021 is no regular bootstrap high-side driver.
--- Updated ---

For the generated Vgs voltage versus supply voltage refer to data sheet diagram 3-2.

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Hi,

I'd say it's a regular bootstrap capacitor (and function)
... but the way it is charged is different (from many other high side drivers), because it has an internal charge pump ... to be able to operate up to 100% duty cycle.

Klaus

Hi,

If you do an internet search on "boost capacitor function" you get a bunch of good explanations.
This "boost" circuit is also called "bootstrap circuit". You may also do an internet search on this.

Klaus
Could you explain whether my understanding of the MOSFET voltages and time constant is correct?
--- Updated ---

MIC5021 is no regular bootstrap high-side driver.
--- Updated ---

For the generated Vgs voltage versus supply voltage refer to data sheet diagram 3-2.
This is where I got confused too. If the supply voltage is 24V from the figure, then the Gate voltage is only close to 13-14V. But when the gate voltage is 13 or 14V, the source will be 24V, right? For the MOSFET to turn on again, the Vgs should be greater than the MOSFETs threshold right? But in this scenario, if the gate voltage is lower than the source voltage, how will the MOSFET turn on again?

A bootstrap circuit typically charges the capacitor to (near) the bootstrap circuit supply voltage.
Then, to turn the MOSFET on, the bottom of the capacitor is connected to the MOSFET source terminal, and the top of the capacitor connected to the gate.
Thus the MOSFET gate will be stay at the capacitor voltage as the source rises to equal the drain voltage (MOSFET fully turn ON), so the gate-source voltage is independent of the original drain voltage.

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