DRV8833 PWM Control ...

Hi Christopher,

I hope the following document will answer your questions:

http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slva321

It is a supplement to our DRV88xx datasheets

Hi Rick,

Thanks for the link.  That certainly makes the whole concept of current recirculation & decay modes more clear but it doesn't specifically address how these modes should be used when running from a PWM signal (for speed control).  So it doesn't really answer my question.

I may just be missing something but it appears that a motor can be driven (i.e. actually moved) using PWM in 2 different methods - slow-decay & fast-decay.  OK, I understand the FET transitions that occur and how the current paths are established when the power is removed from the IN pin(s).  But since I'm driving the motor with a PWM signal and there will be HIGH & LOW conditions continuously while the motor IS moving, which of the 2 modes should be used?  And what impact does the mode choice have on the physical operation of the motor?  When actually STOPPING the motor, I do want to use slow-decay (to provide as much brake action as possible), so my plan is to set Ain1 & Ain2 HIGH to force a STOP condition with brake.  But when I'm moving the motor forward should I use:  a) Ain1=PWM & Ain2=0 (fast-decay) or b) Ain1=1 & Ain2=PWM (slow-decay)?  And how will this choice physically affect the operation of the motor?   Again, my understanding of current recirculation while the motor is supposed to be moving (under PWM conditions) has not been helped much by the datasheet or the additional app note you provided.

Hopefully I've made my point of confusion more clear.  Appreciate any help you can provide.

Hi Christopher,

I hope you have understood the basic difference between fast and slow decay modes as explained in application report SLVA321 suggest by Rick. Choosing one over other really depends upon end application requirement together with type of motor is used i.e. DC or Stepper. Below I am providing further explanation of above application report to give little more insight of differences between the two modes:

Refer to figures 3 and 5 of application report and assume for time being that FETs and Diodes are ideal switches with zero on-state drop and motor load current is in continuous conduction mode, the basic difference between the two modes are:

In fast decay mode, motor voltage is bio-polar in nature i.e. it high level is equal to VS and low level is –VS whereas in slow decay mode motor voltage is uni-polar in nature i.e. it high level is equal to VS and low level is zero.

Implications with DC motor:

Fast decay mode results in bi-polar input causes more ripple in motor current as compared to slow decay mode. Due to bi-polar nature of input voltage, effectively zero voltage would be applied to motor at 50% of PWM duty cycle in continous conduction mode. This is another drawback because PWM Duty cycle resolution is split in two, less resolution for speed control & power consumption at all times with 50% duty cycle at zero speed.

Implications with Stepper motor: Typically stepper motors are used with micro-stepping scheme which mostly require sinusoidal type current profile to be supplied to motor. With this type of application, its best to use both modes called as mixed decay mode and this is well explained in section 2.3 of application note.

In summary, I would suggest to go for slow decay mode with DC motor and mixed decay mode with stepper motor.

I hope above explanation clarifies your doubts, let us know if you need any more details

Best Regards

Milan Rajne

Hi Cristopher,

For Brushed DC Motor control you want to use slow decay for the same reason Milan mentioned. When you are in slow decay mode, the current ripple will be less than on fast decay mode. This implies you will get higher average current on slow decay than on fast decay, which at the same time means you will get more torque on slow decay than on fast decay. If you play with a DC motor driver which has a MODE pin (like the DRV8800/01) you will see that switching from slow to fast decay makes the motor move slower. This is because you just made the available torque smaller and the motor cannot move as fast.

There is also a missconception that slow decay means brake which raises an eyebrow: "How can my motor move faster when I am braking it all the time?" Slow decay is not brake. Slow decay uses the same FET topology than brake, but they are two different scenarios. On slow decay, both same side FETs are ON and the current decays slowly. The key work here is "current decays". Once current reaches zero, then Brake occurs and this is when you see the BACK EMF taking over and pushing a current opposing to the initial current.

Why is this important? Well, because for slow decay mode to work properly, your PWM frequency has to be fast enough to make the current regulation continuous. If you let the current reach zero, then you will be hitting brake, in which case speed regulation is impossible. I am just mentioning this as an observation as I am positive your PWM frequency is high enough for this not to happen, but it is a good thing to keep an eye open for.

Hope this helps!

[EDIT] BTW, you will also notice that you need a higher PWM duty cycle to start the motor if you use fast decay. Again, this is because the average current on fast decay is less, which in turn means you have less torque.

Thanks for your help.  It's getting clearer ... good to know that running slow-decay is NOT the same as brake mode (this was a confusing point to start).

Yes, I am using the DRV8833 to control a set of 6V DC Brushed motors (with 1:75 Gear Ratio).

In general, when running with PWM, average current is reduced (regardless of decay mode) so motor torque will reduce as the PWM signal gets lower.  And this becomes more apparent when running in fast-decay since the average current is even lower than slow-decay.  Are these points correct? 

Regarding PWM frequency, at what level does this cause issues when running in slow-decay?  My PWM frequency is currently around 500Hz.  Is there a recommended PWM frequency for the DRV8833?  If the PWM frequency gets too low, what performance impacts would a motor give that could indicate a frequency problem?  Are there any issues with running the PWM frequency too high?

So, when running in fast-decay it may be necessary to increase the PWM value to get the motor moving (due to lower average current) and then it can be reduced back down again, correct?  How quickly can the PWM value be lowered after getting the motor moving?  Is it best to kick-start the motor at full PWM and then reduce quickly or just provide a bit higher PWM value to get it started?

Cristopher,

You are correct. Applying a PWM with a duty cycle smaller than 100% will in fact reduce motor speed/torque. Whether duty cycle affects speed or torque, however, is quite the religious discussion. I subscribe to the "faith" in which PWM duty cycle is the same as voltage scaling. And voltage scaling is the same as speed scaling. In other words, if you apply a 50% duty cycle while your application's voltage is 6V, then the motor sees the equivalent of 3V.

Now, some people will argue that the motor moves slower because it is seeing less current. This is also true! The fact is you can't separate one from the other. The motor moves slower because the voltage is less. But at the same time, the current must be less (V still equals I/R in this portion of the universe) and hence the torque is less. Instead of getting confused I subscribe to the thumb rule that voltage equals speed and current equals torque. Period! We can philosophize it all we want on next Friday's happy hour, but if you want to keep your sanity, just stay with one rule and move on.

Luckily, PWM frequency is not so much a topic for religious discussion. In this case, the laws of physics will be quite definitive. There will be a frequency so low in which case your winding current will no longer be continuous. This is where you will actually start to brake your motor on a periodic basis. My impression is that 500 Hz should be fine, but the components will have the last word. I can't fully answer the question because at the end it is all a factor of your application voltage, motor winding inductance and resistance, and motor speed. What are the relationships?

The higher the voltage, the higher the current increases. Since you are in slow decay mode, then the current discharge is irrelevant of application voltage.

The smaller the inductance, the quicker the current increases and decreases.

The smaller the resistance, the quicker the current increases and decreases.

The faster the motor is moving, the smaller the current gets. This is because the BACK EMF fights the power supply, in which case less current build up can happen.

Why is all of this important? Because at the end how much the current increases during the H Bridge TIME ON and how much the current decreases during the TIME OFF will be dependent on all of these variables. If the frequency is high enough, then the motor never gets much of a chance to fully discharge. But if the frequency is too low, then the combination of all of these parameters may render the application in danger of suffering from discontinuous current. Whether his is a problem for the application or not, is your call, although in my experience it is a pretty bad behavior which must be avoided at all costs.

The best way to see whether the current is in danger of becoming discontinuous is to either measure the current with a current probe or measure shaft actuation with a shaft encoder. What you should see at the shaft is a fairly bad case of torque ripple caused by the continuous ON/OFF tugging.

If you go with too high a frequency there are also issues. Not necessarily with the motor, but most likely with the power stage as the switching losses increase. My favorite switching frequency is anywhere in the vicinity of 30 KHz. It is above audible levels and it is not high enough to generate too much driver heat.

With regards to your last question, it is possible that you can start the motor with a larger duty cycle and then take it down to a lower level. I guess this depends on the motor and its mechanical construction. I would just use slow decay, though. YOu can start with 100%, but what most people do is ramp up the PWM duty cycle. How steep the curve is will determine how agressive your acceleration is. If you are closing the loop, you can ram the duty cycle for a small period of time in which the system is under open loop conditions and then let the close loop take over. Just suggestion, though. Whatever works for your should suffice.

Great questions!