DRV8811 Problems

Hi Steve;

would like to understand a little more about your system. please contact me directly at jcg@ti.com and we can explore further.

best,

juan garcia

The device was faulty; I replaced it with a new one which seems to work fine.

I do have problems with heat dissipation though. I'm only running with a 1A current limit and the IC gets hot really quickly; I have a top PCB thermal pad and it's connected to the bottom ground plane (approx 10 square cm) with VIAs. I basically can't run it without a fan. Does TI have a reference PCB layout for either a 1A or 2A current limit?

I also built a circuit using drv8811 driver and I’m having the same problem mentioned above. The current limiting doesn’t work.

I tried to set the trip current to 0,5A with a value of 2V reference voltage and 0,5R sense resistors. The current has been limited just by my motor’s resistance. I tested with a resistive load as well, with the same result. Different sense resistor and different reference voltage changes nothing.

I also replaced the driver with a new one, but the problem is still alive.

Any idea about this?

Thanks,

Balazs

Hi Balazs,

The DRV8811 works around a topology called Current Regulation in which a current that would be higher, is regulated down to something smaller. Per example, imagine your 3V stepper. It was made to get some current, say 1A, if 3V are applied. This pretty much tells you that the motor resistance is 3 Ohms.

Unfortunately, to get a 3V drive is not really that easy, plus is not a good idea anyway. You want the voltage to be way higher so that you can charge the inductance pretty quick and move the stepper faster. So we will want the voltage to be something like 24V, or possibly 36V. Of course this raises a little problem called I = 36V/3Ohms = 12A. Say your power supply can in fact deliver the 12A, will you want your stepper to sink said current? If you want to fry some eggs, perhaps. At 12A, that motor will burn itself in no time considering it is rated at 1A, not 12A. Hence, we must somehow take the 12A current down to 1A.

This is very simple to achieve by chopping the current and I will not go into current chopping details, but will point out that it is almost like PWMing the voltage, which in essence results in a reduced current. So now you have the benefit of an increased current charging rate at the winding, without the peril of a heinuous current.

That being said, this mechanism works to take a current down in magnitude, not to take it up. In other words, it is like a current Buck Converter, not a current Boost Converter. It is not a current source. If the system is such that the current is less than the ITRIP you have configured it to (and remember ITRIP = VREF/(8*RSENSE), then ITRIP events will never be generated and the current will not be regulated to the ITRIP value. It is like a voltage regulator that takes a 9V from a battery and steps it down to drive your 5V MCU. Well, when the battery is at 4.5V, so will the MCU be at 4.5V. If it were a Buck/Boost converter, however, then the MCU would always be at 5V, until the battery is completely drained.

So as you can see, there is nothing wrong with the chip. All you have to do is either increase the voltage or change the motor to an unit with less resistance and you will see the effects of the current chopping taking place.

Hope the info helps. Best regards,

Jose Quinones

Sorry, you probably misunderstood my problem.

The system's supply voltage (VM) will be 24V. At this value, the current is much greater than it should be (my reference voltage is 2V, Rsense is 0.5 ohm -> Itrip=0.5A .) I tried with less voltage, like 9V while I measured 900mA. So it seems, that the current limiting not working.

Hi Balazs,

Sorry for my missunderstanding, so let me see if I got it now. You have configured the ITRIP to 0.5A but yet you are getting 900 mA with 9V or you would be getting 2.4A with 24V, Correct? In other words, the system looks like a 10 ohm resistor to the power supply and that is the current limiting factor, but not the current regulation engine as we would have expected.

The first thing I must ask is, how are you measuring this current? Also, what system is this in, is it one of our EVM's or your own board? It almost seems like the VREF is somehow something rather large, or the SENSE resistor is smaller, which would then result on ITRIP being configured to much more than 0.5A.

Can you double check the reference voltage is in fact 2V? I know you mentioned it was on your previous message, but I want to make sure we eliminate this variable. I would also make sure the resistor is 500 mOhms as well. If it happens to be 50 milliohms, this would definitely explain why the current is not being limited as now the ITRIP would be 5A, instead of 500 mA. In fact, by looking at these numbers, it seems more plausible the problem is the RSENSE and not the VREF, as the VREF would need to be scaled accordingly and you are already at 2V. For the ITRIP to be so off because of VREF, then the voltage would need to be much higher which in essence would damage the device.

One last detail. Current regulation does not work with resistive loads. Remember every time we enable the H Bridge we must apply a time period in which current is ignored, better known as TBLANK. This is used to remove spurious noise caused by the FET/diode switching during H Bridge enablement and avoid the system to recognize an ITRIP event when in fact there is none. On the DRV8811, the TBLANK is selected by the C at the RCx pin, but usually, TBLANK is about 1 and 2 us. Hence, during this time you will see VM/R current if your load is resistive and not inductive.

This is to be expected as current choppers are meant to be driving inductive loads, not resistive. Once TBLANK expires, though, I would expect to see the current being chopped accordingly.

Please let me know if this still does not solve your problem and we will go from there. Best regards,

Jose Quinones