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# DRV8306: Set the right R_ISEN value and power rating

Part Number: DRV8306

Hi,

In regard to the previous question, it was explain how to select the R_ISEN value (Based on the 0.25 Vlimit).

From that I derive that the power rating of the resistor should be I_max^2*R_ISEN.

If I look at the selected R_ISEN (CRE2512-FZ-R007E-3) from the DRV8306EVM (R29), it is a 7mOhm resistor of 3W of power.

I_limit will be about 35A (0.25/0.007) so P will be 35^2*0.007 = 9W.

In a stall situation or high load, the current can continue to be at the higher limit and the resistor will fail.

Am I missing something?

Thanks,

Tomer

• Hi Tomer,

If the current * RSENSE result in a voltage that exceed Vlimit =  0.25V for longer than tOCP_DEG then the DRV8306 will go into cycle-by-cycle current limit mode to limit the phase current so that the phase current * RSENSE is under the Vlimit value.

Regards,

Joshua

• Thank you Joshua,

I still don't understand it.

Let's look at another scenario where the current does not reach 35A so V across Rsense will be lower than 0.25V.

Say I=30A so V=30*0.007=0.21V, this will not trigger CBC so it can run continually at that level.

The power across the Rsense will be P=30^2*0.007 = 6.3A which will exceed the power rating of the resistor.

Can you explain this to me please? Am I missing something?

• Hi Tomer,

I will need some time to look into this. I will get back with you by Thursday.

Regards,

Joshua

• Thank you,

I appreciate that.

• Hello Tomer,

Your calculations care correct, at 30 A the R29 shunt resistor on the DRV8306EVM would fail. The DRV8306EVM is limited to support a maximum of 16A due to the max current rating of the J5 connector (691101710002) used for VM. This limits the power that the shunt resistor will experience to 16^2 * 0.007 = ~1.8W.

If you were to create a board which could support 30A max, you would need to make sure the shunt resistor's power rating is greater than 30^2 * Rsense.

Regards,

Joshua

• Got it,

Thanks you, I will use several resistors in parallel for that.