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# DRV8306: Set the right R_ISEN value

Part Number: DRV8306
Other Parts Discussed in Thread: CSD18514Q5A, CSD88584Q5DC

Hi,

I would like to understand how to calculate R_ISEN?

In the EVM it is set to 0.007Ohm but I do not know the pulse by pulse current limit based on it.

From what I can understand is that Vlimit = 0.25V and and in the datasheet the Winding sense current range is -20A to +20A so R=0.25/40 = 0.00625 so 0.007 was chosen.

Are my calculations correct?

Why are we measuring +/- range? The direction of the current in the lower side should always be positive.

Thanks,

Tomer

• Hello Tomer,

I would like to understand how to calculate R_ISEN?

Your calculations are correct if you are attempting to determine a Rsense value for a Pulse-by-Pulse limit of 40A. On the DRV8306EVM the Pulse-by-Pulse current limit can be calculated by using the following equation: Vlimit / Rsense = 0.25V / 0.007Ω ≅ 35.7A.

The -20A to 20A winding sense current and other design parameters from Table 4 of the datasheet were not used as design parameters for the DRV8306EVM and only serve as an example parameters not specifications. Just to provide an example of a difference between Table 4 and the EVM, the EVM uses CSD88584Q5DC MOSFETs not the CSD18514Q5A MOSFETs listed in the table.

Why are we measuring +/- range? The direction of the current in the lower side should always be positive.

You are correct that for commutating the motor the direction of the current will be always positive, but during braking the current maybe negative which is why the internal current sense amplifier has the ability to measure negative current.

Regards,

Joshua

• Got it.

Thank you.