MCF8315A: Current Measurement Discrepancies on MCF8315 Evaluation Board

Noam yuzuk

Part Number: MCF8315A
Other Parts Discussed in Thread: MOTORSTUDIO, , MCF8315C

Tool/software:

Hi,

I am working with the MCF8315 evaluation board and would like to use the I_LIMIT function.

I have observed that while it does limit the current, there is a discrepancy between the actual current and the current measured from the power supply. Could you clarify why this difference occurs?

Additionally, the current readings I measure differ from those displayed in the MotorStudio application. What could be causing this discrepancy?

Best regards,
Noam

5 months ago

0 Venkatadri Shantaram84961 5 months ago

TI__Mastermind 23662 points

Hi Noam,

ILIMIT is for motor coil current.

It is per phase peak current value.

The BUS current is estimated value from three phase which will have some error due to dead time.

Can you provide more details about measured value and motor studio register?

Thanks and Best Regards

Venkatadri S

0 Noam yuzuk 5 months ago in reply to Venkatadri Shantaram84961

Prodigy 45 points

Hi,

Venkatadri Shantaram84961 said:
Can you provide more details about measured value and motor studio register?

Addrerss 400h from data sheet

can you remind me what is the calculation of base current ?

this is the calculation from my software

BusCurrent = ((float)read_data / 134217728.0f) * 0.78125f;

0 Noam yuzuk 5 months ago in reply to Noam yuzuk

Prodigy 45 points

sorry the address is 410h

0 Venkatadri Shantaram84961 5 months ago in reply to Noam yuzuk

TI__Mastermind 23662 points

Hi Noam,

Base current for MCF8315A is 6.25.

I missed to ask about the device marking, is this MCF8315C or MCF8315A?

Thanks and Best Regards

Venkatadri S

0 Noam yuzuk 5 months ago in reply to Venkatadri Shantaram84961

Prodigy 45 points

the evaluation is MCF8315A but who is connected is MCF8315C - very strange

0 Venkatadri Shantaram84961 5 months ago in reply to Noam yuzuk

TI__Mastermind 23662 points

Hi Noam,

The EVM are replaced with MCF8315C now.

We have improved few features and recommend to use MCF8315C.

Thanks and Best Regards

Venkatadri S

0 Noam yuzuk 5 months ago in reply to Venkatadri Shantaram84961

Prodigy 45 points

Hi,

Still i don't understand,

Does the MCF8315C will give a better accurate current ? or i will get the same current with error ?

Or am i doing something wrong ?

How can i fixed that ?

There is 100 - 200 mA error.

Noam.

0 Venkatadri Shantaram84961 5 months ago in reply to Noam yuzuk

TI__Mastermind 23662 points

Hi Noam,

We suggest using MCF8315C if any of the features which I showed in the table important for your requirement.

But current measurement accuracy is same.

The Bus current is estimated from three phase current, and the accuracy will be affected due to dead time and sampling.

Is it possible to share numbers you read against the power supply actual current? I will try to explain the difference.

I am mainly interested in the current magnitude at which we are seeing 100 to 200mA, by how much percentage we see deviation?

Thanks and Best Regards

Venkatadri S

+1 Noam yuzuk 5 months ago in reply to Venkatadri Shantaram84961

Prodigy 45 points

hi,

i did an experiment , i operated the blower at 20 to 100 % and the measure from the power supply current and from the i2c result,

and this is the result :

Deviation [%]	error[A]	measurement [A]	MCF i2c[A]	cmd [%]
52.17391304	0.024	0.046	0.022	20
29.41176471	0.05	0.17	0.12	40
16.66666667	0.08	0.48	0.4	60
13.04347826	0.15	1.15	1	80
5.714285714	0.08	1.4	1.32	100

i will be glad if you have an explanation for this data.

Noam.

0 Venkatadri Shantaram84961 5 months ago in reply to Noam yuzuk

TI__Mastermind 23662 points

Hi Noam,

As mentioned earlier, device estimates bus current by sampling three phase current.

The device idle current (when not driven) is not measured which will be close to 25mA + some misc load on the board is also not measured.

There will be some error due to dead time also and hence we see higher error during lower speed and error reduces at increased current.

Thanks and Best Regards

Venkatadri S

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MCF8315A: Current Measurement Discrepancies on MCF8315 Evaluation Board