Steve Hardy wrote:

>Another untested option is to use a charge pump to charge a capacitance
>connected between the driver and the gate.  This requires an initial
>'priming' of the charge at power up, and the occasional transition to
>maintain the charge:
>
>              C1                      --- Isink
>         -----||----------------      |
>         |                     |   ||--
>Drvr ----+->|--+->|--->|--->|--+---||
>               |  N-1 diodes       ||--
>              ---                     |
>              --- C2                  |
>               |                     GND
>              GND
>
>( >| represents a diode with drop of 0.5V)
>
>In general, V(C1) will be Drvr(hi) - Drvr(lo) - N*0.5  where N is the
>total diode string length.  The FET must be chosen so that the
>available gate swing is enough to bias the fet fully off and on.
>The capacitance should be chosen so C(FET) << C1 << C2.
>
>A similar network is used for the high side (P-channel) device.
>
>On the subject of charge pumps, if you have a spare PIC pin, you could
>use it to drive a charge pump (3 stage, to get 12-14V) or even a tiny
>SMPS - much like what your LT1111 is doing.  This supply is then used
>to drive the gates via a suitable level shifting transistor.  Yes, this
>_is_ totally off the planet, but no-one ever said motor control was
>easy - look at the prices of motor control chips!

 I particularly like the ingenuity of this approach - but, with due
deference to Steve, I suspect the above circuit will be slow in turning the
motor OFF and that discharge resistors (R1,R2, below) will be needed, plus
more regular pulsing to maintain the ON voltage. A capacitor from gate to
source *might* also be necessary.

              C1                      --- Isink
         -----||----------            |D
         |               |         ||--
Drvr ----+->|--+----+->|-+->|--+---||
               |    |          \  G||--
              ---   \          /      |S
          C2  --- R1/        R2\      |
               |    \          |      |
               |    |          |      |
               +----+----------+------+-GND

 Lots of scope for experimentation here!

 Cheers,   Murray




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