EVAL6482 TCC and IGATE parameters

pfournier · ‎2013-05-20

Posted on May 20, 2013 at 20:37

Let me set you up on my situation before I ask my question.

I am experimenting with an EVAL6482 with my design. I am trying to reach 2900 steps/sec with my mechanical hardware. I manage to get to 2200 steps/sec without a real problem. The next jump to 2900 causes it to run then stall. One of my thoughts was that the MOSFETS were not on long enough to reach the current. So I tried adjusting TCC in GATECFG1 register. I thought increasing the value would keep the gate drive on for longer. Interestingly there seemed to be little to no effect UNLESS I brought the TCC value down to 0 or 1, 125nS or 250nS. At those levels the 6482 tripped the OCD! This was totally unexpected, I figured it would just not run. To get to TCC=2 I had to turn the TVAL_RUN and TVAL_ACC vary low. I increased TCC all the way to the maximum and nothing noteworthy happened. I finally put a scope on one of the gates and found that the gate voltage width was ALWAYS 684 uS no matter what I set TCC to. Am I looking at the wrong thing here?

BTW, I originally chose IGATE of 96mA and TCC of 625nS because gate charge is 55nC and those were 2 settings that got me at least 55nC. If I had chosen a lower current I would have had to choose a longer TCC.

#l6482

Enrico Poli · ‎2013-05-24

Posted on May 24, 2013 at 14:59

Hi,

Your original setting of the TCC and IGATE values was correct. The meaning of the two parameters is exactly what you have stated: the total gate charge supplied to the MOSFET gates.

Increasing this value through the TCC, no benefits can be obtained. Decreasing the TCC the gate charge is reduced and the RdsON of the MOSFET is higher. A further reduction of the TCC could cause the gate charge to to be so low as not to turn on the MOSFET at all. In this case the OCD event is triggered (the OCD measures the Vds voltage, so if the MOSFET is not turned on the Vds is equal to VS).

About your speed limit issue: are you testing the motor with a load connected to the shaft? Driving ''unloaded'' stepper motors could cause resonance issues.