Part Number: INA233
Customer is designing INA233 on power calculation.
Would you please provide comment for the following calculation is correct or not?
0xD4 => 0x2800
0x4A => 0x2710
0xD0 => 0x4827
0xD2 => 0x00FB
Customer read back 0x96=22327(10)
So the power is calculated as below
then X=22327/20000 = 1.11635W
Thanks for using our forums. From the information provided it looks like you are calculating the real-world value correctly. Is your customer's value different from what they are expecting? If so, it would be helpful for us if you provide the expected current and bus value as well as the current and bus values the device reads. The shunt value would also be helpful.
Patrick Simmons, TI Sensing Products Applications Support
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In reply to Patrick Simmons:
Would you please review the design and calculation with comment and suggestions?
please refer to the actual measurement data. it seems the error is very big.
V1=18.9V, I = 1.2A
P = 22.68w
this is the schematic.
In reply to SHH:
I am looking into your issue. Unfortunately our IT has blocked installation of the INA233EVM software for the moment, while I wait to fix that issue. Perhaps I can get a few more details from you and your customer to determine whether the part is broken, improperly programmed, or have a communication issue. To rule out whether its broken, If your customer supplies the part with an external supply, does it show the supply sourcing well above the typical quiescent current (mAs)? If not, its likely a programming issue. If your customer reads register 0xD4 after writing, do they actually read 0x2800?
Please refer to customer's read back register value and let us know your comment.
I think I found part of the issue. Your slope coefficient, m, for power should be calculated as 1/(25*current_LSB) = 800. You can then shift that left 1 place and use R=-1. While that should be considerably closer to what you are wanting, from my calculations that would be 28W. With an 18.9V bus and 12mV Vsense I get 22.8525W. I think it would be worthwhile to look at both the Read_VIN and Read_IIN registers to find where the discrepancy is with your measurements.
can you please confirm the calculation is correct or not?
for Read_VIN and Read_IIN, I will check with customer.
You are on the right track, but there is one additional step the datasheet recommends to avoid possible rounding errors. That is maximizing m. Below, I will summarize all the steps so that it is clear for you and any person who looks at this post in the future.
Calculate current_LSB: (typically calculated from maximum expected current, however is calculated from CAL in this case)
CAL = 0x2800 = 10240
Rshunt = 0.01ohm
Current_LSB = 0.00512/(CAL*RShunt) = 50uA
Calculate m for power:
m=1/(25*Current_LSB) = 800
Shift (maximize) m and get R:
if 1000*m>32767>100*m, R=-2, new m = m*100
else if 100*m>32767>10*m, R=-1, new m = m*10
else if 10*m>32767>m, R = 0
10*m = 10*800 = 8000<32767: m =8000, R=-1
Calculate the real-world value, x:
Y=22327x=1/m(Y*10^(-R)-b) = 1/(50uA)*(22327*10^(-1)-0)=27.90875W
thanks for reply. Would you please provide comment for the following questions?
Here are the two different project with INA233 read back 0x88, 0x89 and 0x96 and actual voltage and current data.
When I convert the values for case 1, I get pretty close to your multimeter measurements. As for case 2, the current value does deviate quite a bit. Is this a different device? If so, you need to check the calibration register and make sure the correct value is there. If the correct value is there, then either there is a problem with the device or there is some parasitic path siphoning off current between the multimeter and the INA233. There are a few ways you can go about testing this. The first would be to swap out the part. If the reading gets better than the device may have been damaged. The other thing you can do is put your multimeter on the low-side of the shunt (assuming your previous multimeter readings were on the high side). If a low side multimeter reading shows lower current as well, then you need to start to look at components around the shunt that may be damaged and are pulling more current than expected.
To further assist you with your debug, I am attaching an excel file with my calculations based on the information you provided above.
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