cooking A3984

[van]

hi
i have been working with A3984 for last 3 weeks.
I have a nema 17 type cube in shape, 5v,0.33ma ,15ohm resistance of each winding stepper motor.i soldered A3984 on general purpose PCB.
i have cross checked all the connections after lots of soldering stuff.

i set VREF=4.0v, RS=1 ohm and supply voltage=10v.i've attached ROSC terminal to VDD for fixed off time of 30usec(default). when i turned on the ckt i saw smoke coming out of A3984.I find out that OUT terminals were shorted by a solder ball which was may be came from my dirty tabel.I clean my PCB and replace the A3984(again solder on general purpose pcb).and also cross cheked the connections.

I put my finger on the A3984 before giving power supply cuz this time i don't wanted to see the smoke coming out of it.i started the ckt with out attaching stepper motor.there was no smoke and no heat.i turned off the ckt and attached the stepper motor and again turned it ON.The motor was moving and i thought it's done.but actually not.the motor was not rotating in one direction it gose forward one step and comes back again no effect of step 2 and 4.i test the OUT pin using DMM and measured the output voltage.the sequence was coming out perfectly.then i measure the current and i found out that only one winding was geting active means only one bridge was able to give current the second bridge was not giving current although it shows voltage perfactly.what can be the cause....................?

i have already crossed cheked the connections.connections of sense resistors are perfect.and my source is of 12v 2amp.After two days when i again start the ckt a3984 was getting hoter in some fractions of a second.i again chek out all the connection all the connection were perfect after so much testing i found out that the resistance between ROSC and VDD terminal is 203ohm only where else it was more than 1k when i fresh solderd it.i disconnect it from VDD and give it ground via 20k of resistor.when i start the ckt everthing was ok but the same problem waiting for me there.i don't know why the hell another bridge is not giving current?.i have also changed the RS resistor to 2ohm so the current limits to 250ma only.whats wrong?

another thing i wanted to ask is in the datasheet of A3984 it is given that
RS=Vref/(8*Itripmax)_ _ _ _ _ _ _(1)
and in the FAQ of A3982 for finding out RS value so that the voltage accrose RS resistance never go above 0.5v it is given that
RS=0.5/Itripmax(ohms law)_ _ _ _ _ _ _ _ _ _ _ _(2)

putting value of RS from equ(1) into equ(2)

0.5/Itripmax=Vref/(8*Itripmax);
Vref=4;
so the Vref will always be 4v.NOW what's the point to give any voltage in between 0 to 4 to Vref? please help...........

[PMinMO]

If I had to guess, I'd start at the sense resistor for the bridge that isn't working. As for the vref, is you use a sense resistor of .25 ohms, with 4V at the vref terminal would set the trip current at 2A. If you adjusted the 4V vref to 2V, then it would be 1A.

[van]

If I had to guess, I'd start at the sense resistor for the bridge that isn't working. As for the vref, is you use a sense resistor of .25 ohms, with 4V at the vref terminal would set the trip current at 2A. If you adjusted the 4V vref to 2V, then it would be 1A.

well thanks for the reply sir.but the problem was not the sense resistors
the problem was a capacitor at pin VREG which controls internal PWM switching ckt.I by mistake put a 22 Pf capacitor instead of 0.22 uF.due to that when ever i tried to enable the bridge output it heats up.i have already disigned a PCB for the A3984 with AVR contoller and it's working great.i have also gone through the microstepping mode the motor turnes very smothly without vibrating and it goes at 955RPM above this i haven't tried.

now there is one problem and that problem is when i stop running the motor it startes humming.I can here a kinda sound which is rotating in the winding of a motor. what is that? how can i turned it off?i can not disable the bridge when not running due to some requirment..

well i guess the answer is hidden somewhere in fixed off delay for A3984 it is ROSC terminal....

By the way thanks for the reply sir.........

[PMinMO]

The sound in all probability is normal. Most chopping stepper controllers have a annoying sound. If the chopping frequency is high enough it may not be heard, but even then sometimes there is a beat frequenct between the two phases that is audable.