How to determine the value of shunt in DRV8305

Hi Shirai,

Selecting the Shunt Resistor should be done with several factors in mind. The output of each sense amplifier can be calculated by:

Vo = (Vvref / k) - (G * (SNx - SPx)

So in order to select you shunt resistor you would need to know what values you intend to use for Vvref, k, and G.

These values can be adjusted using their respective SPI registers or different package variations, but the defaults are:

Vvref is Package Dependent, DRV83053 = 3.3 V and DRV83055 = 5 V
k = 2
G = 10

For this example I'll use the DRV83053, so Vvref = 3.3 and we'll keep the default values for simplicity.

Substituting these values into the above equation:

Vo = (3.3 / 2) - (10 * (SNx - SPx)

For best common mode rejection SPx should be connected to ground, so we'll assume that value is 0:

Vo = (3.3 / 2) - (10 * SNx)

Using a 3.9 mOhm resistor and your stated current spec of 40 A max, Vo would be:

Vo = (3.3 / 2) - (10 * (40 * .0039)) = .09 V

The power dissipation of the 3.9 mOhm resistor would be:

(.0039 * (40^2)) = 6.24 Watts

If each of the three half bridges will be using a Shunt Resistor than the power will be generally less than this because each resistor may only see 1/3 of the current. If all three half bridges are tied to one Shunt Resistor then this power dissipation will be continuous and the part selected must be able to withstand that.

If you look at page 37 of the DRV8305 Datasheet Table 18 and Table 19 show the various SPI registers associated with different G and k values.

The main consideration for Gain selection is the settling time of the Shunt Amplifier. A larger gain will result in a longer settling time but a larger gain will allow for a lower required ADC resolution and a smaller Shunt Resistor meaning less power dissipation.

A final consideration is the range of the ADC used to sample the output of the Shunt Amplifier. If the ADC range is 0 - 3.3 V selecting a properly sized shunt resistor will allow for full utilization of the available range.


Hope this helps!

Phil Beard
Motor Applications Team