L298H limitations ?? is anyone familiar with the h-Bridge

[BRADFORDSCOTT]

Hello,

I am working on my first CNC project. i have designed the stepper motor driver with a pic micro controller (16f1933) . and iam using the L298 h bridge.

The 16f1933 has 5 ccp modules, so iwas going to use 2 of them for micro stepping and 2 others ones for current wave chopping.

I have these motors

NEMA23 425oz/in 2.8A Stepper Motor ¼” Dual shaft (KL23H286-20-8B)

425 oz In. Hybrid Motor
1.8° /200 Steps Per Rev.
2.8 Amps Current Per Phase ( Bipolar Parallel)
8-wire Bi-polar or unipolar, NEMA 23 Frame

http://www.automationtechnologiesinc...H286-20-8B.pdf

and i have an Allen Bradley 1604xl ,24Volt ,10 Amp supply to drive them

I was going to run the motors in a bipolar paralleled configuration
which the rated voltage is 4.17Volts

4.17/24 =.17 , .17 *100 =17%

i was planning on constantly driving a PWM 17% duty cycle at 22kHZ, so that the average voltage would be of the rated voltage, but i would reap the benefit of faster rise times in current.

i ran my connections directly from the pic , to the L298h enable A and B pins. as soon as i try to run a constant PWM signal into the enables , the motor does not work .

( i have also tested on a 12v stepper, if i tie the enables of the 298 high there is no problem driving the stepper)

Also well driving the 12V stepper i switched to the PWM enables and scoped the output to the motor , i receive the PWM signal at low voltages 0-5V on the DC power supply , but anything around 6V and above the PWM signal disappears.

. am i passing the Limitations of this chip., by trying to have a continuous PWM signal feed?

Looking at the schematic the enables tie to an and gate configuration does the L298 have switching speed considerations?

the 298 says you can run 38 Volts at 4 amps.

thanks for The Help in Advance

Brad

if anyone has any other methods i welcome the feedback

[doorknob]

I have no experience working with the L298, so I can't address your questions directly.

However, maybe you can find some answers in the L297 application note at http://www.technologicalarts.com/myfiles/data/AN470.pdf starting on page 10 (where it discusses PWM considerations for driving the L298 in some detail).

[BRADFORDSCOTT]

i have went over them in detail but it still does not mention frequency that the bridge can switch at

[doorknob]

There is a Commutation Frequency spec in the L298 datasheet (that lists 25 kHz typical and 40 kHz max at load current of 2A), however I have not seen a precise definition of that parameter. It would seem that 22 kHz is in the ballpark.

[H500]

There are many potential pitfalls in both the hardware and software. Do you have an accurate schematic?