Talk about the upper and lower tubes of the buck circuit_DC-DC BUCK bootstrap circuit

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Talk about the upper and lower tubes of the buck circuit_DC-DC BUCK bootstrap circuit [Copy link]

What is a bootstrap capacitor?

The DC-DC BUCK chip has a pin called BOOT, some called BST, as shown below:

How does the bootstrap capacitor work?

The following is the block diagram of the DCDCBUCK chip. The NMOS above is called high-side

When the high-side MOS tube is turned on, SW is VIN, and SW is

The above is the general principle of DC-DC BUCK.

C in the figure is the bootstrap capacitor. When the low-side MOS tube is turned on, SW is grounded to 0, and the voltage on BOOT is provided by BOOT Charge. If it is 5V, the capacitor is charged. When the low-side MOS tube is turned off, the high-side MOS tube is turned on because the high-side Vgs>Vgs(th). Therefore, the high-side MOS tube can be turned on. As the high-side MOS tube is turned on, the voltage on SW will become VIN.

If this C is not added, then when Vgs

How to choose the rated voltage of the bootstrap capacitor?

Generally, the maximum value of BOOT to SW will be given on SPEC, which is 6.5V as shown in the figure below, so generally a capacitor with a withstand voltage of 10V/16V can be selected.

Bootstrap Resistor Selection

In fact, resistors can also be added to the bootstrap circuit, generally called BOOT resistors. The function of the BOOT capacitor is to pump the BOOT pin voltage to a higher voltage than SW when SW is at a high level, using the characteristic that the voltage across the capacitor cannot change suddenly, to maintain the on state of the high-side MOSFET.

Adding the BOOT resistor and the BOOT capacitor forms an RC charging circuit.

The size of the BOOT resistor determines the switching speed of the high-side MOSFET. Generally, the larger the BOOT resistor, the slower the high-side MOSFET turns on. At this time, the smaller the spike on SW is, and the better the EMI characteristics are. The smaller the BOOT resistor, the faster the MOSFET turns on, and the larger the spike on SW is, so sometimes RC is reserved on SW for ground absorption.

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It is rare to see a resistor in series with the BOOT capacitor. This has a disadvantage. The voltage drop on the resistor causes the supply voltage drop seen by the driver inverter to drop suddenly. The unstable voltage difference may cause the levelshift output to flip incorrectly and the highside to frequently switch malfunctions. If it is simply to limit the opening speed, reduce the driver size, or add a resistor in series on the gate, which will not affect the power supply level of the driver.