🚀 go-pugleaf

tod@cco.caltech.edu (Tod Edward Kurt) writes:
>
>Hey everyone,
>
>	Here's my dilemma:  I've got a voltage that varies (say from 0-5v) and
>I'd like to have this voltage control a DC motor via Pulse-Width Modulation.
>I've got the actual high-current motor driver (MOSFETs) done, I just need some
>pulses for it.
>	It seems like there should be a chip that does all this, has an
>adjustable PWM frequency, and provides a way to drive the motor both forward
>and reverse (2 seperate outputs, one for forward, one for reverse)
>	Any ideas?  Surely this must have been done thousands of times.  All 
>help appreciated.  BTW, this is for an Electronic Speed Control (ESC) for an
>RC car.
>					Thanks,
>						Tod
>tod@sandman.caltech.edu
>tkurt@vlsi.caltech.edu

There are several ways of doing this, but the most straightforward that
I can think of is using a DAC and a binary "up" counter to generate a
voltage sweep (very accurate sawtooth wave) covering the range of voltages
you are interested in and with a period equal to the desired PWM frequency.

Use a simple comparator to compare the sweep signal to the variable voltage
and use the output to drive the MOSFETs.  If you find that the motor speed
isn't as linear over the control range as you like, you can play around
with modifying the top and bottom voltages of the DAC (this effectively
changes the minimum and maximum duty cycle of the PWM)

I know Radio shack used to sell a real low cost 7 bit (I think) ADC which
was really an 8 pin chip with a DAC, a 7 bit counter, and a comparator
(Just what you want!).  All you gave it was the Voltage to be measured,
a clock reset signal, a clock signal, and power.  As an output, it gave
you the comparator output.  I don't have the exact part number, but it
was something like ADC080?.  So, this chip and a 555 should be all you
need to do what you want.  If I can find the spec sheet for this ADC,
I'll e-mail you (the original poster) the exact number.  Now that I
think about it, it might have even included an ap note on using it as
a PWM generator...  If so, I'll include that info.

Cheers,
 Dean Reece (deanr@sco.COM)