INA226: Current monitoring circuit

Thanks for your reply and making it clear to me that we can measure current only at high side of the load or low side of the load. But the position of the resistor itself look like in series to the load. If it says shunt then I think of parallel, if it says series series then I would think of a resistor in series. Kindly explain how the resistor in series to the load become a shunt resistor. Thanks in advance. 

Thanks Louis for your reply. It make the concept clear. 

I am wondering about the value of the resistor. I am not sure if it is ok to ask here because this is a design parameter. 

We have a DC to DC converter that provide 4 Amp current with the supply voltage of 5V. There are some LDO connected to the output of LDOs which supply the power to the several ICs. We actually want to measure the total current at the output of the DC to DC converter. We can accept few tens of mV drop in the supply voltage which is not a problem. Any suggestion on the value of the shunt resistor and of what power rating.   

One more question. How do we measure the supply voltage at the load ? I guess only the voltage difference across the shunt resistor is amplified by the differential amplifier which is used to measure the current, given that the resistor is known. 

John,

Today is a U.S. holiday but I will get back to your question when I return to the office tomorrow.

Louis

John,

John B said:

Any suggestion on the value of the shunt resistor and of what power rating.

I suggest you watch our video specifically on selecting shunt resistors linked here. It will walk you through all of the care bouts when selecting he appropriate shunt resistor for your specific application.

John B said:

How do we measure the supply voltage at the load

If you look at the diagram on the first page of the datasheet, you can connect the supply voltage of the load the the VBUS connection of the device, then you'll be able to monitor that voltage by reading the VBUS register.

Please let me know if you have any other questions.

Louis

I started calculating shunt resistance by defining the peak power dissipation across the shunt resistor which is 200 mW. 

The nominal voltage and the nominal current through the load is 4 A and 5 V. The maximum current through the load is 5 A. 

Let's calculate the shunt voltage given the power is 200  mA and the max current is 5 A. 

P = V x I 

V = P / I 

V = 200 mW / 5 A

V = 40 mV

This shunt voltage 40 mV is well within the input shunt voltage range according to datasheet. 

Now calculate the value of the shunt resistance. 

V = I x R 

R = V / I 

R = 40 mV / 5 A

R = 8 m Ohm 

The value of the shunt resistance is calculated as 8 m Ohm. 

Now we need to calculate the Current LSB according to equation (2) on page 15/39 of the datasheet. 

Current LSB = Maximum Current / 2^15

Current LSB = 5 A / 2^15

Current LSB = 152 uA/bit 

Assuming the last two bits are noisy and not very stable. Or we do not want very high precision. Let's ignore the last two LSBs. We multiply by 4 to get new Current LSB. 

Current LSB = 610 uA/bit 

Is my calculation are ok until now ? 

Now we calculate the Calibration Register value using equation (1). The values of Current LSB and the R_Shunt need to be converted to A/bit and Ohm. 

CAL = 0.00512 / (Current_LSB x R_Shunt)

CAL = 0.00512 / (610 /1000/1000 x 8 /1000)

CAL = 1049 decimal 

The resolution of Shunt Voltage Register is 15 bit ? The LSB is 2.5 uV ? 

Does it means that full scale reading will be 2^15 x 25 uV = 81 mV approx ? 

One last question. How do we calculate junction temperature ? Which power dissipation we use to calculate the junction temperature ?

If the power dissipation across the shunt resistor is 200 mWatt for example. Do we need to use 200 mWatt multiplied by junction-to-ambient thermal resistance and add to ambient temperature to calculate the junction temperature ?  

For example junction-to-ambient thermal resistance is 171.4 degC/Watt. The power dissipation across the shunt resistor is 200 mWatt. 

The junction temperature will be 

Tj = Tambient + junction-to-ambient thermal resistance x Pd

Tj = 40 + 171.4 x 0.2

Tj = 74.2 degC

Is that correct ? 

John,

Yes, these calculations are correct to estimate your junction temperature.

Thank you very much for your reply. 

There is still one thing I am not sure about it. I understand that increasing the value of shunt resistor will cause more heat dissipation across the shunt resistance and also crease the shunt voltage that may improve the current measurement accuracy but at the of more heat dissipation across the shunt resistor. 

Will it also increase the junction temperature of the INA226 ? I am asking this question because the shunt resistor is located physically outside the INA226. So the question is, increasing the shunt resistor value will also increase the junction temperature of the INA226 ?