Kevin wrote:
> They are small DC Motors 1.5v to 3v.
> 0.18 - 0.25A at no load 0.70A +- 15% at max effic.
> I am using 4AA NiMh batteries to power the board. I am
> feeding th SN754410 with the raw voltage. The chip drops 3V
> as per the data sheet.

Yikes!  No wonder it gets hot.  You've got about a 5V supply.  You certainly
don't want the driver elements dropping 60% of the voltage.  Your putting
more power into the drivers than into the motors.

> Right now I am not using PWM I just
> have a logic high on the enable pin for each driver.

So you only need to switch the motors on and off, not run them a partial
power?  If so, that's even easier.  Use a logic level N-channel FET as a low
side switch.  Since this is not constantly switching, the switching
transition time doesn't matter much.  Just drive the gate directly from a
PIC pin, source to ground, drain to the motor, and other side of motor to
+supply.  Don't forget the reverse diode accross the motor and put a small
ceramic cap as close as possible directly accross the motor to reduce
emissions and other nasties.

A small and cheap FET like IRLML2502 would be fine here.  It comes is a
SOT-23 package, but that doesn't matter since it won't be dissipating any
power.  700mA thru 45mOhms is only 22mW.  Many other FETs would be fine too.
The right NPN could be used too.  This is really not a very demanding job
for the pass element.


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