Hartung, Greg wrote:
>   I have been trying to a get 4 of these working in an H-bridge also, but
>perhaps I've been lead astray.  I was lead to believe that the gate voltage
>needed to be 5v+Vds.  Is 5v enough to turn a 540 on completely?
>

Greg,

From what I can tell from the datasheet, the answer is yes. The Vgs
threshold is 4v max, so if you tie the 540 source to gnd, then 5v out of
a PIC should do it. However, if you are also using a 540 for the upper
transistor in the bridge, then you will not be able to turn it on without
a larger voltage.

Better probably to have p-chan MOSFETs in the upper level, with "source"
wired to Vpower - ie, mirror image to n-chan MOSFET below - and both
gates wired together. Then the same 5v in will turn the upper one off, and
the n-chan in the lower position on - and vice versa for 0v in. This is
exactly how the output stage in any CMOS chip works - here the
PMOSFET/upper - NMOSFET/lower h-bridge is basically emulating same.

Note - this will only work for Vpower = 5v. If you want to run the
h-bridge at higher voltage, then you will need a level-shifter -
std npn inverter with collector pullup to Vpower and collector
also tied to the MOSFET gates should work, I think.

Also, you might look into using "logic-level" MOSFETs, with
Vgs threshold only ~2v, rather than 4v.

cheers,
- dan michaels
www.oricomtech.com
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