Need Help!- ta8435

[untenops]

Hi Everyone
Hopefully I'm in the right place to post my question. I need some help with the TA8435. I have a 3 axis stepper control board that is based on the TA8435. But the little info I have on it is not answering my question. So Ive been looking at the data sheets for that chip.
http://www.toshiba.com/taec/componen...//382/3537.pdf
But for some reason I can not wrap my head around the current sensing mentioned on page 21 of that data sheet. Is this chip capable of limiting the current supplied to the stepper motors? If so how do I make use of this? Does pin 10 need to be held low for this to work? What size resistors do I need to use on pins 18 and 21? Right now the board has 5 ohm resistors there. If I'm reading the data sheet right I think that limits current to 1 amp. Maybe? Here is a little more information on what I'm trying to do. Right now
I have stepper motors hooked up that are 12 ohm and a 16 volt power supply. So I should be drawing round 1.3 amps. My meter is reading anywhere from .8 to 1.3 amps total from the power supply with one stepper active. Ok? And it works fine but I need to use some steppers with a bit more power. So Ive ordered one to try out that is 3.6ohms. Well at 16 volts that over 4 amps, to much for the 8435 to handle. But if it limits current I should still be able to use it, I think. Please help.

[vger]

Looks like that chip will allow you to select 2 current levels via pin 10. The voltage dropped across the Rnf is compared to either .8 volts or .5 volts depending on the logic level at pin 10. To set the current you would use the formula Io = Vref/Rnf. That is Current output equals Vref (set by logic level on pin 10) divided by Rnf (the value of the resistors at pins 18 and 21. You can turn the formula around to be Rnf = Vref/Io. So with pin 10 at a logic low (for Vref of .5 volts) and to get 2 amps output, you would use 0.25 ohm resistors for Rnf at pins 18 and 21. Switching the logic level at pin 10 to a high for 0.8 V Vref would jump your output current to 3.2 amps. This is probably done so you can use a higher current while driving (moving) the motor, and switch to lower current while resting. The thing is, this chip controls the current to the motor by monitoring the motor current via the voltage developed across Rnf and switches the power off as the current goes over the set limit, then switches it back on again with the next oscillator pulse, and off again as it goes over the limit again. With this type of controler, the power supply voltage can be quite high, but the current through the motor windings will be limited by the chip.

Hope that helped
Steve