UC2625: UC2625

Hello Ian,

Thank you for posting to the MD forum and for the information. Have you been able to probe EA_OUT to check if its out of spec when the device is hot? and how much does the speed differ when the system gets hot from what it was originally set to be?

It is possible that they could drift at the same rate but I do not have much information on this at the moment. If you are thinking about creating a larger PWM signal then it might worth it to check if this yields similar results to your current setup. If the whole system gets hot and not just the UC2625 it could degrade some of the performance of the external passive components also leading to the shift in speed you are seeing. Have you noticed if the rest of the system is getting hot?

Additionally, if this is a new system I would recommend perhaps going with a newer part, the UC2625 is an older generation device and using a new one might include features that work better with your system.

Looking forward to your response!

Best,

Isaac

I am able to probe EA in + which is buffered right into EA_OUT, that signal drops to around 150mV when hot (400mV cold) and the speed changes from about 1100RPM to 1400RPM.

Our whole system does get pretty warm, 85C ambient plus all internal rises inside the unit.

Are you aware of any DC characteristics of the error amp and comparator. Might be that when they get hot they draw some current and add to offset voltage, which could push the ramp away from EA_OUT?

Any ideas on better chips out there?

Hey Ian,

Thanks for the info here, I will look into this tomorrow to see if that could be the case.

Can you give me a couple of specifications on your application so that I can make some recommendations?

Voltage, current, sensored/sensorless control, control method (trapezoidal, sinusoidal, FOC), integrated FETs, number of PWM signals needed. This should help me start making suggestions.

Best,

Isaac

Sure thing

We are running 28V, 10A, 200W motor, sensored, trapazoidal, 6 discrete FETs low sides get the PWM signal

Hey Ian,

Not sure if the 28V is taking voltage margin into consideration for your system, but based on the information I would recommend taking a look at DRV8320 and DRV8323, the main difference between the two is that DRV8323 has three built in current sense amplifiers. They also have a DRV832xR which indicates that there is a built in buck regulator.

Let me know if you have any questions here.

Best,

Isaac

Hey Ian,

I forgot to mention that I am still looking into the comparator drift with temperature, I need to consult with the team since this device is older so I appreciate any patience here. Thank you!

Best,

Isaac

Hello Ian,

Just some questions for the debug, also if you could provide a copy of your schematic that would be extremely helpful.

1. To confirm a few things,
   
   1. Is it possible to provide an oscilloscope capture of H1, H2, H3 and TACHOUT on the same frame to confirm whether the TACHOUT is decoding the hall signals and generating the proper PWM waveform output?
   2. Is the part holding a stable temperature during operation of the test, or is it changing significantly under operation?
      1. Looking into the possibility that the device performance is drifting with temperature.
   3. What is the waveform on pin#27 (E/A Out)? 
      1. We want to see if we can localize the problem to a particular functional block of the IC
2. Miscellaneous debug
   1. Does this issue persist for only the speed setting you are currently using (Pin #1 voltage)?
      1. Have you tried some other speed settings to see if the issue persists for 1V, 1.5V, 2V for example?
   2. Have you seen this across multiple units or was the speed change only seen on one unit? 

Best,

Isaac

Hi Isaac,

I am not in a spot where I can hop back into the lab as the stuff is being used for another test.

However, I can answer some of these. The issue is definitely thermal related, if we command 1100RPM at room temp it is good, but if we heat the controller up it will drift to 1400RPM then blow on the chip and it pulls right back to 1100RPM

It does persist across all development units that we have.

Seems that at 2000RPM when we heat up then there is no issue, but if we start commanding lower speeds, then heat it up the motor speed will drift away from commanded speed.

Hey Ian,

Thanks for that info, do you mind sending the snippet of your schematic so that we can see where E/A In (+) is connected to?

Best,

Isaac

We had to make a decision and found a workaround. Thanks for your help Isaac

I believe there was an impedance mismatch between the PWM comparator inputs, dropped the PWM input pin impedance and it started to work appropriately. This was a random shot in the dark but it seems there are some DC characteristics (Vos, Ios,Ibias) that impedance mismatching could have been aggravating some of those values.

Awesome this should be helpful to others encountering this issue thanks Ian!

Best,

Isaac