In the BUCK circuit, we often see a capacitor connected between the SW and boot pins of the chip. This capacitor is called a bootstrap capacitor. There are several questions about this capacitor.
What is the use of bootstrap capacitor? Why do some bucks have bootstrap capacitors and some bucks do not have bootstrap capacitors?
Omar Posted on July 27, 2020
Take the chip MP1484 as an example, as shown below:
When DC-DC is working: SW pin voltage is connected to GND when M2 is on, and VIN when M2 is off. SW voltage is the same as power input VIN. The Vgs of the upper tube M1 is controlled by the driver. In order for Vgs to be turned on, Vgs must be greater than the turn-on threshold Vgsth of M1. Because the negative terminal of the driver's power supply has the same voltage as the source input Vin when M2 is disconnected, the driver's power supply must be greater than Vin.
In general, the energy of the bootstrap capacitor should be able to meet the energy provided when the entire M2 is turned off and provide energy for the next startup.
Kendall Posted on July 27, 2020
The self-contained capacitor is a synchronous switching power supply, which uses an internal MOS tube to replace the external freewheeling diode for freewheeling. The upper and lower tubes are internally driven by the upper tube.
Emmanuel Posted on July 27, 2020
The bootstrap capacitor is to reduce ringing.
