LM5060: Accuracy of current sense - Iratio (Isense/Iout-en)

Hi Beat Jetzer,

Thanks for reaching us on E2E!
Can you please share your application use case, end equipment and target accuracy requirement to check whether LM5060 fits

Best Regards,
Rakesh

Hi Rakesh,

this is a snippet from our schematic. R1 (Rsens) could change, if we are able to increase the current through the FET:

Our use case:
- we need a high side FET (IAUT300N08S5N014) circuit, that is self protecting (no damage of the FET if a short circuit is done)
- a very slow 40A fuse (car fuse) will protect the circuit for long time overload
- the FET doesn't have to be damaged, if a continuous current of 40A is flowing
- the driver (LM5060) doesn't have to switch off the FET if a continuous current of 40A is flowing (better would be a current of 45A, having a safety clearance to the FUSE, protecting for errorneous shut-off)
- temperature range: -20°C .. 70°C ambient
- not more than 2.5W power loss over the FET's (parallel rdson = 1mOhm @Tj=145°C) -> max. 50A -> max accuracy: 45A +/-5A.

Hi Rakesh,

Thank you for your fast response and the proposal of a more accurate driver!
Another important requirement is low operating voltage. It is between 6..7V and should go up to 60V. Besides, the LM5060 has a much better VGate/VIn behavior.

Is the Vds tolerance of the LM5060 also including the Iratio tolerance? If the Vds tolerance leads to a current sense tolerance of 7A (7mV, 1mOhm Rdson -> 7A) and the Iratio tolerance is about another 15%, then the total tolerance (without Rdson tolerance) is about 30%.. Is it like that or is the Vds tolerance measured at the pins Sense and Out?
It would really help, if the total tolerance band of the relevant parameters would be shown in dependency of the chip temperature.

It seems, that I have to optimize the thermal coupling of the FET -> increase the maximum current through the FET -> being able to cover the 30% accuracy of the current sense.

Or do you have other suggestions of driver? Please let me know!

Thank you and best regards,
Beat Jetzer

Hi Beat Jetzer,

On the min operating voltage requirement, LM5060 is the only option we have.

Vds tolerance is coming from the offset of Vds comparator and it does not include Iratio tolerance. Since, you are not using Ro resistor, Iratio will not effect your accuracy as per Eq (21) in datasheet.

Please use the attached sheet for accuracy calculations. Feed the Rds(on) variation with temperature to estimate the accuracy.

LM5060 Isc accuracy cal.xlsx

Best Regards,

Rakesh

Hi Rakesh,

Your hint, that Voffset is measured directly at the pins is very helpful.

But I'm a bit disappointed about the error calculation you've done in the excel sheet. It is totally wrong:

 

You calculated (uA x Ohm) - V ... the units are not in the same range -> the Voffset doesn't have any influence in that case!
I think, that the usage of Voffset min or max is not correct for min/max calculations, but I'll prove it and I'll upload my version of error calculation. Please have a look on that as soon I've upload it.

Hi Beat Jetzer,

You are correct. I have overlooked on the units..
Please share your calculations as soon as it is ready.

Best Regards,
Rakesh

HiRakesh,

here is the Excel sheet with the modified accuracy calculation. Please have a look.

BR,

Beat

8540.LM5060 Isc accuracy cal.xlsx

Hi Beat Jetzer,

Thanks for correcting the units.
When the units are adjusted, we get the same values and it is expected from this device.

Best Regards,
Rakesh