DRV8412 Cycle-by-cycle Current Limiting

Tyler,

In latched mode, there is one comparator that trips within 250ns upon exceeding the limit. This is called LEVEL1 and set via the external OC_ADJ resistor.

In CBC mode, there are two sets of comparators. The first set (LEVEL1) responds in 400-450ns once the threshold is reached and brings the current down by disabling outputs...this is the same level as latched mode, but the timer is increased to allow the current to be brought down through duty cycle limiting.  If there is a hard short on the output, aLEVEL2 comparator will trip and latch a fault within 250ns....it is quicker than LEVEL1 to protect the device from a hard short.  LEVEL2 is approximately 6A greater than the LEVEL1 threshold set via the OC_ADJ resistor.

With a hard short, the current will shoot up much higher and much faster with a direct short and this trips a LEVEL2 fault and results in a latched /FAULT even if the device is in a CBC current limiting mode.

Thanks for the great explanation Ryan.

I’ve been running into some issues testing the CBC current limiting on the DRV8412 with the EVM. To run my test, I disconnected the control card and used a function generator input instead. I also changed the mode jumpers to (M1 – low, M2 – low, M3 – low) and lowered the current limit to approximately 3.0 A. I found that regardless of what I was attempting the DRV8412 would latch instead of CBC at the current limit.

I then changed the mode jumpers to (M1 – high, M2 – low, M3 – low) and the DRV8412 then worked with CBC current limiting instead of latching. This is opposite of what the data sheet says. My main question is if this issue is an EVM issue for the DRV8412 itself?

Attached is an image of the jumper configuration as well as a scope plot of the CBC current limiting in this configuration. For the scope plot, yellow – PWM_in, green – PWM_out, purple – current measured to one of the 2 motors.

Tyler,

Something funny is going on.  But, you had me nervous so I hooked it up on my bench and ran a brushed DC motor.  I stalled it with some pliers and observed a  /FAULT on the board (LED6 turns RED) in the bottom of the board.  If I put it in M1 = M2 = M3 = LOW, then I get no /FAULT signal, but the device goes into CBC.  CBC mode does not report a /FAULT, so this matches my assumptions.

Have you measured the voltages on the Mode pins?  I am wondering if there isn't something wrong with your EVM at this point.  

Something is definitely unsettling Ryan. I've measured the voltages on the mode pins and they are what they should be (3V3 on high, 0 on low). As far as the fault LED is concerned, I am getting similar results to you but my observations on my scope are different. When M1 = M2 = M3 = LOW and the motor is stalled, LED6 does not turn on and the motor latches as opposed to CBC limiting. When M1 = HIGH and M2 = M3 = LOW and the motor is stalled, the LED6 activates and the motor goes into a CBC current limiting state instead of latching. I re-ran my test several times, with different configurations and motors just to be sure and this is what I am seeing. So this is probably an issue with the EVM? or, since LED6 is tripping with CBC at the same time, is this a problem with the driver itself?

Regards,

Tyler

Tyler,

The fact that the /FAULT pin is cycling low, but the outputs are still switching in very interesting to say the least.  Does the /FAULT line stay LOW while the outputs are switching?  

In the M1 = M2 = M3 = 0 case, are any of the outputs switching?  The interesting thing about CBC is that if you are holding one of the inputs LOW and PWM'ing the other input, the motor can still stall even in CBC mode if the over-current detector on the non-switching output (that corresponds to the input you are holding LOW) triggers.  It can look like a latched case, but it really is not.  If you pulse the input that you are holding low...as if you are changing direction on the motor...than the device should recover without cycling /RESET.  I wonder if this is what you are seeing.  

Are you using the supply that came with the kit or a bench supply?  The supply provided with the EVM can current limit and if voltages are dropping to the chip than that might explain some of this.  

Ryan,

The fault line stays low when M1=M2=M3=0 but not when M1=1, M2=M3=0.

In M1=M2=M3=0 case, when the OC latches, none of the outputs are switching.How could the non-switching output that is being held low trigger the OC latching? I have not tried it on the other PWM input so that might be the issue. That doesn't however explain why we are seeing CBC working when M1=1, M2=M3=0.

I am using the kit supply so that might be an issue as well.

Don't burn out trying to resolve this issue as we have moving on to another driver for our application as we require a different max current for each motor output.

Tyler

Tyler,

No matter what I do, I can't seem to reproduce this on the kit I have.  So, most likely something happened to the IC, but I have no idea what could happen to explain what you are seeing.