HPWM problem .. pls help!

[NavMicroSystems]

The frequency limit for the L293D is 5kHz !!

see Datasheet !

[Samuel]

ohh thanks! i just checked the L293D data sheet and it doesnt talk about the limit of 5KHz .. But i guess it makes sense.

A duty cycle of anything below 100 doesnt push my motors. So the range for my duty cycle would be between 100 and 255? And the PIC with a 20Mhz OSC cannot go below 1221Hz, and my L293D doesnt go above 5KHz. So my frequency range would be between 1221Hz and 5000Hz? Much better .. now that i've narrowed down to something ...

I like ure signature "There are only 10 types of people: Those who understand binary, and those who don't ..."!!

[anj]

i just checked the L293D data sheet and it doesnt talk about the limit of 5KHz

My datasheet ( Unitrode ) doesnt give this limitation either.
16KHz gives a period of 62.5uS. Fig1 in the application info shows rise/fall times and these are less than 0.5us, so the chip shld cope with 16KHz easily

Re getting hot, at low duty cycle ( ie just getting started or creeping ), the motor is not very efficient and can draw a lot higher than normal current. For this, the datasheet is indispensable, as these chips do get hot and they do need thermal heatsinking.
I have done a lot of testing with "small" motors and PWM and found the best low speed performance was at very low period ( I go down to 28Hz for one motor ). I have never tested with larger motors so cant help there.
If you still want to use a 20MHz osc and HPWM but get a much lower output freq, you can run the signal through a deecade counter typ chip, and just pinch the relevant output that suits yr needs.
Or i just found this, never tried it but ut looks interesting
http://www.pbpgroup.com/modules/wfse...hp?articleid=6
Andrew

[NavMicroSystems]

L293D Datasheet

[anj]

Gday
Interesting.
I have an original Unitrode datasheet, and a quick search brings up the current texas one http://focus.ti.com/docs/prod/folders/print/l293d.html
Neither of these specify a 5KHz limit.
They mention doing their rise/fall time measurements at 5KHz, but thats all.
Based on the actual rise/fall times ( your link vs texas link ), they appear to be close, so i am at a loss to understand the difference.
I certainly have run my 293D at higher than 5KHz before.
Dunno

[Samuel]

Well right now after playing for a whole afternoon with different conbinations of duty cycle and frequency, the HPWM 1, 135, 3000 seems to work well for me .. at high frequencies my motors start doing funny stuff .. I feel this may have to do with the how the 293D is setup. I have my two PWM lines, one going into each of the enable. And then 4 control lines, 2 for each motor. There is another way of setting up, which can reduce the number of lines by using an Inverter. Since this setup is working for me, and i have enough ports on my PIC controller, i'm not gonna try that for now ... I just have to find someway to keep the 293D cool .. Someone suggested to use some kinda paste .. tooth paste??? And then what?

[anj]

Someone suggested to use some kinda paste .. tooth paste???

Thermal transfer paste ( heatsink paste ). And a big heatsink.

And then what?

Go for a dedicated H Bridge using N and P channnel mosfets.
Using one PWM channel and some pulldown diodes ( from between the gates current limiting resustor and the gate, back to a PIC pin ), you can control where the PWM goes, hence dirn.
Andrew