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Original question:

DRV8832-Q1: Motor Drivers Forum

DRV8832-Q1: Unable to remove current sense limit by shorting ISENSE to ground. Also, more errors in datasheet?

Will DeCook
Prodigy 30 points
Part Number: DRV8832-Q1
Other Parts Discussed in Thread: DRV8832

Tool/software:

I have a motor drive circuit using the DRV8832QDGQRQ1 with a 0.15Ω current sense resistor (I_LIMIT=1.333A). I want to increase the current limit, but don't have the appropriate resistor on hand at the moment, so I decided to short ISENSE to ground in order to completely remove the current limit. 

Per Section 7.3.5 of Rev C of the datasheet:   
"If the current limit feature is not needed, the ISENSE pin may be directly connected to ground."

I added a wire jumper shorting out my existing 0.15Ω resistor. With this setup, I get a fault immediately upon enabling the motor drive. This is with the same motor load that was working fine with the 0.15Ω resistor. Why would this fault be occurring with ISENSE shorted directly to ground? Is the datasheet correct about this functionality?

I'm starting to think that we are inducing an OCP fault by tying the ISENSE pin to ground. 

Per Section 7.3.6.1 of the datasheet:  
"Overcurrent conditions are sensed independently on both high and low side devices. A short to ground, supply,
or across the motor winding will all result in an overcurrent shutdown."

Where and how is OCP being sensed? There are two diagrams right next to each other in the datasheet that show differing accounts of the OCP sense connections. One shows the OCP sense input at the high side of each channel (effectively the same thing), the other shows one OCP sense input at the high side and one at the low side. If OCP is being sensed on the low side where shown in Figure 7, then does shorting ISENSE to GND meet the "short to ground" condition that causes an OCP fault?

Section 7.2, Functional Block Diagram

Section 7.3.1, Figure 7:

  • 0
    Prodigy 30 points

    I see in this post that someone was experiencing similar behavior with a DRV8832. Unfortunately, that issue was not addressed in the thread, which has now been locked.

    https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1061239/drv8832-motor-diver-issue/3939285?tisearch=e2e-sitesearch&keymatch=drv8832#3939285

  • +1 in reply to Will DeCook
    TI__Mastermind 30915 points

    Hello Will,

    1. OCP is sensed on each FET as mentioned in the datasheet. The block diagrams referenced are showing this only at a high level. OCP will be reported by the Faultn pin. With OCP, the device will remain disabled until VCC is removed and re-applied. Is this what you were experiencing? 

    2. Current limit circuit is independent of the OCP sensing and detection. Current limit has internal 200 mV reference which helps size the low side bridge external sense resistor for proper current limit. When the current limit is reached the internal PWM duty cycle will be automatically reduced to limit the current to the set value. While this will also be reported by the Faultn pin the bridge will not be disabled as it would the OCP. Recovery does not require power cycling.

    3. After shorting the current limit resistor to GND there will be no current limit imposed - the ITRIP comparator will never trip because the voltage on ISENSE will be < 200 mV all the time. With the limit disabled, if the load you have had inrush current that will exceed IOCP specifications of the device it will trigger an OCP which was not happening with the current limit while using the 0.15 Ω resistor. You could measure the load current inrush spike with a current probe and an oscilloscope.  

    I think this may have been what you were experiencing. In order to avoid OCP trip you should select a ISENSE resistor to limit the current to < 1.3 A the minimum specification for OCP to guarantee device operation without OCP under all operating conditions. Setting 1.333 A could be marginal and may fail with some devices that may have the spec closer to 1.3 A and still compliant with the datasheet.  

    Regards, Murugavel 

  • 0 in reply to Murugavel4637
    Prodigy 30 points

    Ah, yes. I think I was probably tripping the OCP on an inrush current spike when ISENSE was shorted to GND. That makes sense. I see now that actually reducing the current limit setting could result in less faults because it will reduce those inrush current spikes and also cause the current limiter to take over and reduce PWM before we hit the OCP limit.