Some problems of DC-DC bootstrap capacitor

Reference resources ：DC-DC Bootstrap capacitor （BOOT） A few questions
This question follows Buck The tubes used inside the chip are related to , If inside 2 All the switches are NMOS tube , Then you need a bootstrap capacitor . But there are BUCK The tube on the chip is PMOS tube , There is no need to produce a ratio Vin High voltage , I don't need to boot capacitance .

DC-DC At work ,SW The pin voltage is M2 When conducting, it is connected to GND Of , stay M2 When disconnected, it is connected to VIN,SW Voltage and power input VIN identical . Upper tube M1 Of Vgs Controlled by the drive , We know , If you want to Vgs Be able to conduct ,Vgs It has to be greater than M1 On threshold of Vgsth. Because the negative end of the power supply of the drive is M2 When disconnected, the voltage is already connected to the source input Vin It's the same , Therefore, the power supply of the drive must be greater than Vin.

stay M2 Conduction time ,SW Got it GND, One end of the bootstrap capacitor is also connected to GND,5V Through the diode to the tube M1 Power supply , And charge the bootstrap capacitor .

stay M2 When it changes from on to off ,SW from GND Turn into VIN, The voltage at both ends of the bootstrap capacitor shall not change suddenly , therefore Boot The end is raised to greater than Vin voltage , Power the drive .

The energy of the bootstrap capacitor shall meet the requirements of the whole M2 The energy provided during shutdown and for the next startup .

Bootstrap resistor selection
In fact, in the bootstrap circuit , Resistance can also be added , General name BOOT resistance .BOOT The function of capacitance is SW At high levels , The voltage at both ends of the capacitor can not be changed suddenly , Will BOOT The foot voltage is pumped to the ratio SW High voltage , Maintain high edge MOSFET The conduction state of .

Joined the BOOT resistance , and BOOT The capacitance constitutes RC Charging circuit .

BOOT The resistance determines the high side MOSFET Switching speed of . commonly BOOT The more resistance , High edge MOSFET The slower you drive , This is the time SW The smaller the peak on the ,EMI The characteristics are good .BOOT The lower the resistance ,MOSFET Drive fast ,SW The bigger the peak on the , So sometimes it's in SW On the reservation RC Absorption to the ground .