BOOSTXL-DRV8305EVM: Current scaling, clarification in documentation

Ananthkumar Selvaraj

Prodigy 120 points

Part Number: BOOSTXL-DRV8305EVM
Other Parts Discussed in Thread: DRV8305

I am using launchpad 28379D + DRV8305EVM + Technic motor dyno setup.

Current scaling

Gain = 10V/V

Shunt resistance = 0.007 Ohm (from data-sheet I assume this is the shunt resistance value.

Measurable current for 3.3 V = 3.3 / (10*0.007) = +/- 23.57 A peak

ADC counts per A = 4095 / 47.14 = 86.8689 counts per A peak

Now question,

document http://www.ti.com/lit/ug/slvuai8/slvuai8.pdf says 0 to 20 A peak currents (in section - Low-side current shunt sense).

Could you clarify from why it is different from the calculated value?

over 5 years ago

0 Adam Sidelsky over 5 years ago

TI__Guru 62720 points

Anathkumar,

The value in the User's Guide is accurate (up to 20A) but the details are left out so let me explain:

For the CSA:

Vout = (Vref - Vswing) - (Vref/2)

Vref of the Boosterpack board is 3.3V, Vswing is min 0.3V.

Vout = (3.3V - 0.3V) - (3.3V/2) = 1.35V

We then divide this by our gain of 10V/V to get 0.135V at the CSA input.

factoring in the sense resistor of 0.007Ω, we get 0.135V/0.007Ω = 19.3A which we rounded to 20A

Hopefully this clears up a few details.

Regards,

-Adam

0 Ananthkumar Selvaraj over 5 years ago in reply to Adam Sidelsky

Prodigy 120 points

Does 19.3 A is peak to peak current? Sorry, still this is ambiguous .
Which means 19.3 A corresponds to 3 V and 4096 ADC counts.
In ADC, 212 counts per A.

Below are my understanding,

My understanding is max positive measurable current is 19.3 A ( 0 - peak) and this corresponds to 1.35 V.

total measurable current (peak to peak) is 42.8571 A (3V, 4096 counts).

Then my ADC count per A is 95.57.

Please clarify.

0 Adam Sidelsky over 5 years ago in reply to Ananthkumar Selvaraj

TI__Guru 62720 points

Anathkumar,

Where does the 42.8571 value come from?

Regards,

-Adam

0 Ananthkumar Selvaraj over 5 years ago in reply to Adam Sidelsky

Prodigy 120 points

Query #1
As per our earlier discussion, max measurable current is 19.3 A.
Please clarify 19.3 A is peak-peak or 0-Peak.

Query #2
For Launchpad 28379D + DRV8305 (with Gain 10V/V),
Please tell me ADC counts per A. (12-bit ADC)

0 Adam Sidelsky over 5 years ago in reply to Ananthkumar Selvaraj

TI__Guru 62720 points

Ananthkumar,

Yes the range is 2 x 19.3 because the current can be 0-19.3 positive or -19.3-0 negative. The 28379D can be configured as 16 or 12 bit. in 12-bit, you should get (2^12)/(19.3x2) counts per amp or ~106.

Regards,

-Adam

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BOOSTXL-DRV8305EVM: Current scaling, clarification in documentation