The charge to replenish the bootstrap capacitor must come from some larger bypass capacitor, usually the VDD bypass capacitor. As a rule of thumb, this bypass capacitor should be sized to be at least 10 times larger than the bootstrap capacitor so that it is not completely drained during the charging time of the bootstrap capacitor. This allows the bootstrap capacitor to be properly resplenished during the charging sequence. This 10x ratio results in 10% maximum ripple on the VDD capacitor in worst case conditions.