Vref of CSENSE in MM9Z1_638

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Vref of CSENSE in MM9Z1_638

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Shaojun
Contributor I

Hi,

i tried to calculate the Vin of CSENSE block with the equation in AN12301(MM9Z1X638 system calibration guidelines) as follows:

微信图片_20210708144034.png

but i cant find Vref of CSENSE in datasheet. Is this the right way to calculate the measured current? And please let me know how to get the correct current if my Shunt Resistor is not 100 uOhm.

THANKS!

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JozefKozon
NXP TechSupport
NXP TechSupport

Hi Xu Shaojun,

please see below an answer from an application engineer I have just received. 

DESCRIPTION

The Vref is 1.25V. We do not specify it as its actually not required for any customer calculations (except GCB).

 

The resolution of the ISENSE is fixed, but a parameter of the selected shunt value IRSEL:

JozefKozon_0-1625740368365.png

To calculate the current measured:

e.g. for COMP_TF-IRSEL = 0b010 -> LSB = 0.1uV   (”IRES” from table above)

 

U_measured = Ushunt = LSB * ADC_Value

I_measured = LSB * ADC_value *  1 / Rshunt

 

You can calculate the current-LSB =  LSB  *  1/ Rshunt

e.g.

for 100uOhm:     0.1uV  *  1/100 uV/A  = 1mA

for 50uOhm:       0.1uV  *  1/50 uV/A  = 2mA

 

If you use a different COM_TF.IRSEL the LSB will change. However I recommend to use the 100uOhm configuration even if the shunt value is not 100uOhm. Current measurement result is not clamping for shunt resistor selection 50 and 75.

With Best Regards,

Jozef

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JozefKozon
NXP TechSupport
NXP TechSupport

Hi Xu Shaojun,

please see below an answer from an application engineer I have just received. 

DESCRIPTION

The Vref is 1.25V. We do not specify it as its actually not required for any customer calculations (except GCB).

 

The resolution of the ISENSE is fixed, but a parameter of the selected shunt value IRSEL:

JozefKozon_0-1625740368365.png

To calculate the current measured:

e.g. for COMP_TF-IRSEL = 0b010 -> LSB = 0.1uV   (”IRES” from table above)

 

U_measured = Ushunt = LSB * ADC_Value

I_measured = LSB * ADC_value *  1 / Rshunt

 

You can calculate the current-LSB =  LSB  *  1/ Rshunt

e.g.

for 100uOhm:     0.1uV  *  1/100 uV/A  = 1mA

for 50uOhm:       0.1uV  *  1/50 uV/A  = 2mA

 

If you use a different COM_TF.IRSEL the LSB will change. However I recommend to use the 100uOhm configuration even if the shunt value is not 100uOhm. Current measurement result is not clamping for shunt resistor selection 50 and 75.

With Best Regards,

Jozef

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