According to this thread: http://forum.allaboutcircuits.com/showthread.php?t=9771

What is mean by "The "bootstrap" capacitor provides a method of using N-channel MOSFETS on the high side of an H-bridge. There is a charge pump in the IC which charges the capacitor about 10v higher than Vdd.

In order to fully turn on an N-channel MOSFET, you generally need to get the gate in the vicinity of 10V higher than the source. Without the charge pump and bootstrap capacitor, when the gate turned on and Vss approached Vdd, the gate would begin to turn off; ie: partially conducting state." ?

Thanks!!!

 

This link may help you - http://www.irf.com/technical-info/appnotes/an-978.pdf

When the high side mosfet is turned off, the bootstrap capacitor is charged to gate voltage on that side. As the mosfet is then turned on the side that was at ground/0volts then starts to rise to source voltage, dumping the gate voltage into the gate. The bootstrap capacitor is used to keep the two voltage levels at the correct voltage offset on the gate. Is this what you were asking about?