when using the TPS40303DRCR, the datasheet recommend using 100nF for bootstrap capacitor. if using 470nF or 220nF, will there be any risk?
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While there is no risk from using 220nF or 470nF. Even if the converter is repeatedly started and stopped, the BP to BOOT driving path is capable of handling the charging current for the 470nF capacitor, there is likely little if any benefit and I would not recommend more than 470nF capacitor.
The SW to BOOT capacitor powers the floating boot-strap power supply for the high-side driver during the high-side FET ON time. The charge drawn out of the SW to BOOT capacitor is approximately equal to the 6.5V gate charge of the high-side FET (Qg). The voltage drop on the BOOT strap circuit each switching cycle will be Qg / Cboot
For a high-side FET with 10nC of total gate charge, the boot-strap will discharge 100mV for a 100nF capacitor, 45mV for a 220nF capacitor or 21mV for a 470nF. The difference in MOSEFT Rdson for these 3 voltages is extremely small. In many cases, even smaller SW to BOOT capacitor can be used, allowing more BOOT to SW ripple, but I would caution against selecting a capacitor with less than 4nF capacitance per nC gate charge. Allowing more than 250mV of ripple on BOOT can increase the peak BP to BOOT recharge current when the SW to BOOT capacitor is recharged as SW returns to ground after the high-side FET is turned off.