Hi, Aman. The UCN5804 is still available at alltronics for $5.50 a pop:
If I was to try something else, I'd use a PIC and a ULN2003 if coil current was less than 1/2A, or a PIC and four TO-220 NPN darlingtons with eight resistors and diodes if more than 1/2A coil current. Here's what one half of one of the stepper coil drive would look like (view in fixed font or M$ Notepad():
Thanks Chris. That is really helpful. I can see that UCN5804 is available right now. But if it is not available 3 months from now my design will fail. I need a design that i can sustain for atleast 2 more years.
O.K., then. If the chip really fills the need, buy a few spares. But if it bothers, how about some more information, like:
Is this a new design, or are you trying to fill an existing IC socket?
Stepper coil current and voltage? Straight coil voltage or L-R?
Maximum stepper speed?
Do you need all the logic capabilities of the chip?
If not, which ones do you need? Read the data sheet carefully.
Thru hole OK or SMT only?
Do you have any capability with PICs or other microcontrollers?
List in order of importance: ckt board space, number of chips, cost.
Beyond the IC itself, there's another issue. What's the rest of your control system look like? If you're using a computer, it would be simple to offload the logic requirement for which phases should be on to the existing computer, and it just becomes a ULN2003 or four darlingtons, depending on current/voltage requirements.
That should be enough to start with. There are other stepper ICs out there, although you probably won't get a one chip solution like the Sprague IC.
Please answer all of the above (middle post in the spaces provided if you want) and you could get an answer that works well.
The new Allegro chips are for bipolar drive, not unipolar. As an extra added bonus, they're only available in SOIC packages, which can be a pain for most newbies/hobbyists.
Microstepping is a lot different than normal stepping. For a discussion on all things stepper motor, see the ultimate web reference, Jones on Stepping Motors:
After you're done reading that, you have to take a good look at your application, and the data sheet for the Sprague chip you originally wanted, and answer some of the questions in the post above. You have to state what you need in order to get a good answer. Otherwise, people will just be thrashing around trying to guess, and lose interest. You and we will have wasted our time.
If all you need is a single-step, low speed indexer/driver, your solution may be as simple as several TTL/CMOS logic ICs and four darlington transistors. But we have to know what you need first to replace the Sprague chip to help you.
Time to learn and work a little to get what you need! ;-)