DRV8301 shunt resistors

Truth in advertising - know little about that specific drive - but have designed similar...

Might peak currents substantially exceed the max continuous currents - thus justifying the apparent "too low" R value? 

Like you - we strive to "choose" our sense R's so that the max, current-driven voltage closely matches "safe, VinMax" of our voltage detecting device.  (usually an analog comparator - our case - due to fast response and ability to function even if SW has "run amuck!")

The 0V3 for over-current protection indeed seems low.  We'd prefer to see a value perhaps ~80% of max_Vin (for the detecting device).  This may be "cut/paste" issue - past/other board copied to this one...  One hopes that 0V3 - if correct - is not subsequently "gained up" - to more normal (~3V) levels.  To me - this "compression" of so critical a signal measure is of great concern...

Hi Mojo,

I believe the sense resistor is scaled in order to account for transient current peaks that you might see when running a motor. By my calcuations the 2mOhm sense resistor allows for peaks up to 150A. This is probably a slight over design to make sure the chip wasn't damaged in a high power situation.

I am confused as to what you are asking for in your second question? The OC_ADC setting is dependent on the Rds on of the external FETs. There is a dedicated circuitry inside of the DRV8301 for over current protection that is unrelated to the amplifiers.

Hi Nicholas,

thanks for the answer.

Maybe I got it wrong about the second part... so how do you set the current limit for the shunt resistors in the DRV8301 ? I mean, can you set it to different value than +-0.3V, or that is fixed, so I need to calculate the right R_shunt when I want to achieve a specific over-current value, that will result in +-0.3V ?

thanks

The shunt resistor feeds back to the differential amplfiers on the DRV8301. The output of the amplifiers feeds back to the controller where it can make speed adjustments...etc. based on these readings. I believe the +/-0.3V spec comes from to what kind of negative potential the DRV8301 pin can handle. This is all completely unrelated to over current protection.

Over current protection is done by monitoring the Drain-Source voltage drop. If you know the Drain-Source voltage drop and Rds(on) you can determine the current through the external FET. The DRV8301 detects the Drain-Source voltage through the SH_X and SL_X pins and based on the setting you configure it will determine if the device has hit OC on the external FETs.

Does this clarrify things?

Hi Mojo,

first, note that for the InstaSPIN software with DRV8301 EVM we are using the external op-amps from the current sense, so you should look at the spec for the TLV devices.

second, the board was originally designed to sense about 82A peak (if I recall correctly) through the DRV830x PGAs. We added the external OPA in Rev D and didn't want to change the sense resistors for those using Rev C for software, so we kept the 2mOhm.

third, the Rshunt is just one factor in the overall scaling.  We have a version _09 of MotorWare going out next Thursday and it will include many updates. One of them is an update to the HW QSG for the DRV8301. I've attached  it here as well, you should review pages 16+ where it discusses the overall scaling through the OPA and how to scale this resolution in hardware and software.

I'm not sure how to answer the last question about the DRV8301 over voltage. I've forwarded to the DRV guys.

Ahh, and I see Nick (DRV guy) has answered. Thanks, Nick.

Hi Nikhil V R,

I would recommend a voltage rating of 50V for those capacitors with a PVDD of 24V.