> Or use logic level MOSFETs and maybe you don't need a driver at all-
> (assuming 25mA of gate drive will do, rather than what a real MOSFET
> driver does) and can perhaps use the BOR in the PIC (pulldowns and 
> gate
> resistors only).
>
> Or, of you do need a driver with logic level MOSFETs, the standard 
> circuit is
> just two complimentary transistors in a dual emitter-follower 
> configuration.
>
> The cheapest solution for relatively low performance is to use a 
> CD4049/4050
> which will drive about 60mA and do the level translation from 5V to 
> 10V
> for less than 2 cents/channel (2500) but watch latchup issues.

If you are running at say 20 kHZ MAX frequency then you have say 25 uS 
on and off times. If however you have 20 Khz frame rate PWM you may 
have pulses in the few uS or faster range.

To switch a FET rapidly you must sweep the appreciable amount of 
charge in and out of the gate capacitance rapidly. To do this in much 
faster than 1 to 10 uS (so pulses in this region are sharp edged) you 
need 100's of mA to amps of drive. The MEAN current is usually 1 - 10 
mA range but the peaks are far higher. The cct i provided and any real 
FET driver will do this. A PIC pin or typical CMOS gate won't. Look at 
the data sheets and do the arithmetic and decide how much current you 
need to get adequately fast switching for your application. A CMOS 
gate or PIC pin will often be 1 to 2 orders of magnitude slower than a 
gate driver.

If you are running at 20 KHz a FET driver will help greatly and at 20 
kHZ PWM frame rate it's mandatory. In production volumes using through 
hole or SMT the discrete circuit is liable to be cost competitive in 
many applications



        RM

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