Voltage Problem with DRV11873

Hi Jon

Could you give the phase to phase resistor of the motor? Also what is your Rcs sense resistor?

to work around for startup failure, below are some possible methods.

Set FS = 0.

Lower the Rcs sense resistor. But keep it >=3.3k.

Lower the three Phase to COM resistors to 300~500 ohm range.

The reason is for some motor, startup current limit may cause big noise which affect the startup BEMF detecting.

Thanks.

Wilson

Motor Application Team

Hi Wilson,

Thank you very much for your replay.

The phase to phase resistor of the motor is 0.5 ohm. The Rcs sense resistor I used is 3.3k ohm. FS is set to zero.

I lower the three Phase to COM resistors from 330 ohm to 220 ohm but still have the same problem.

Is it safe to remove the sense resistor to reduce the effect of the chopping on the BEMF voltage since at 6 volt I reached 480Hz (4100 rpm) with 0.98% duty cycle at 0.4 Amp been drawn from the supply?

Thanks for the support.

Hi Jon

The phase resistor is very small. There will be too much startup current chopping noise. It is not safe to lower the Rcs less than 3k. BTW, I think you want to say 98% duty other than 0.98%, right?

You motor seems have too high power to the capability of DRV11873. Could you give your final (target) application conditions, including MAX VCC, MAX Ivcc, MAX speed?

I will check further to see whether there is any work around of the startup failure issue. Will get back to you later.

Thanks.

Wilson

Hi Wilson,

>>  I think you want to say 98% duty other than 0.98%, right? Yes.

The phase to phase inductance of the motor is 0.05 mH.

The final (target) application conditions are MAX VCC = 10 volt, MAX Ivcc = 2.0 Amp, MAX speed = 7000 rpm.

Thank you very much for the support.

Hi Jon

Besides the startup issue, 2A is just hit the MAX current capability of DRV11873. Also you must have good thermal PAD layout for the thermal dissipation.

To this 0.5 ohm ohm motor, we must have the current limit function for startup.

There may be a little tricky method for the startup issue. Let's call the three resistors as R1, R2, R3 which are just connected to U(R1), V(R2), W(R3) pin. We can make it unsymmetrical as R1=520 ohm, R2 = R3 = 475 ohm. This will give a offset on COM at certain commutation steps which reduce the noise affect to the zero crossing detect. It is not 100% effective with regard to motors, but it is true that there are successful story of this method.

Please let us know the result if you tried.

Thanks.

Wilson

Hi,

I'm getting a similar problem where I can only operate the DRV11873 between 5 - 6.2 V. Are you just directly feeding U, V, and W into the COM input pin with some resistors in series? (My BLDC Motor doesn't have a COM port)

Hi Dennnis

If you don't have COM wire, you can use three same value resistors(500~3k 1%) forming a star connection with the center as the COM and the three ends to U V W.

You said that you can only operate DRV11873 at 5 to 6.2V. What your issue on other voltage? startup failure? Locked?

how about your motor phase resistors? and your FG speed range during 5 to 6.2V?

Thanks.

Wilson Zuo

Motor Application Team

Wilson,

I implemented the pseudo-COM circuit (390 ohm resistors) and managed to get up to around 8.0 V. Above this, the motors suddenly lock up/die. I'm not too sure why.

On that topic, is there anyway to disable lock protection? That's my biggest problem right now. If I set the duty cycle below 30%, the chip will also lock up because it is driving a heavy motor and cannot rotate. (The motor is usually locked, and not in motion like a fan).

Best,

Dennis

Hi Dennnis

What is your FG frequency just before locked?

If FS = 0, FG should be < 450Hz

If FS = 1, FG should be < 950Hz

Other wise, the motor will be locked.

Thanks.

Wilson Zuo

Wilson,

I'm not sure but I can take a look later today. Is there any sequence or procedure to disable the lock protection? Or perhaps escape the lock protection once the chip enters that sequence.

Best,

Dennis Li