Another solution to drive an N-channel MSOFET with the load going to GND is to use a hi/lo gate driver such as IRS2001, LM5101, FAN7842 (all come in an 8-pin packages). They provide sufficient gate drive current and will allow you to PWM at pretty high frequencies - up to 1MHz depending on the application. I typically use them at 5-50KHz. Also, because they have hi and lo side drivers, they are very useful in bridge type situations using N-channel MOSFETs only.