LMP8646: Inaccurate output voltage

Atharva Karaguppi

Part Number: LMP8646

I wanted to use this IC as a current sensor so just for preliminary testing I used a sense resistor(Rsesne) of 0.5 ohms and gain resistance (Rg) of value 49.9K. The supply voltage given to the IC was 12V and the common mode voltage (Vcm) to the input terminals was 6V. I passed a current of 0.4 A. From this I calculated that the output voltage (Vout) would be 1.996 V however I was getting an output voltage of just 0.13 V.

I tried changing the currents to different values however I never got the expected value. The value that I got was always less than what I was expecting.

over 6 years ago

0 Jason Bridgmon over 6 years ago

TI__Genius 14825 points

Atharva, can you share your schematic? We'll look a little more in depth in the morning, but if you could share just that bit around the current shunt device, we'd appreciate it.

0 Mayrim Verdejo over 6 years ago

TI__Genius 15760 points

Hi Atharva,

Thanks for using TI E2E Forums. I did some calculations with your application parameters and you are correct. With Rg=49.9k ohm, Rshunt=0.5 ohms, Vs = 12V and Load = 0.4A you should measure 1.996V in the output (Vout). I also ran a simulation on TINA and got the same results, see below.

For troubleshooting, please measure the voltage on the input pins of the device (between IN- and IN+) to make sure you are getting 200 mV. You can share your schematic with me, I am happy to review it for you.

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

DRV8412 +LMP8646+LP38512.brd

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

DRV8412 +LMP8646+LP38512.sch

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

I have attached the board and schematic files of the PCB that I made. It has 2 more ICs on it DRV 8412 and LP 38512-ADJ.

0 Atharva Karaguppi over 6 years ago in reply to Atharva Karaguppi

Prodigy 140 points

I have made the PCB on EAGLE software.

0 Mayrim Verdejo over 6 years ago in reply to Atharva Karaguppi

TI__Genius 15760 points

Hi Atharva,

Please send me the schematic on pdf format. The Altium file you send me seems to be empty. Also, please measure the voltage between the inputs of the device (as I mentioned on previous reply) and let me know if it is 200mV or not. Refer to this training video for debugging and tips as of where to probe.

0 Mayrim Verdejo over 6 years ago in reply to Mayrim Verdejo

TI__Genius 15760 points

Hi Atharva,

I was able to get your schematic using Eagle. My apologies, Eagle and Altium have same extensions.

I will need further clarification from the circuit. Why is there 2 headers for load? The headers pins are not labeled so I am not understanding the circuit. Also, is the IN- tied to Rsense? There is no nodes on the schematic so it is not clear to me if there is a connection.

Our typical circuit should look like the image below. Can you please confirm that your Rsense and Load are connected this way? Do you have a block diagram? Please confirm the Vsense value as well.

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

The load+ and load- are actually tied to IN+ and IN- respectively.
Yes, the IN- pin is tied to the sense resistor and to the load- pin as well.The load shall be present between load+ and load-.And before load+ I shall place the battery or the voltage source.

Yes I have confirmed the value of Vsesne as well.

In the board is there presence of parasitic resistances in the path of the sense resistor which might be affecting the effective value of the sense resistor and hence resulting into inaccurate outputs ?

0 Mayrim Verdejo over 6 years ago in reply to Atharva Karaguppi

TI__Genius 15760 points

Atharva,

Thanks for the reply. Did you get to collect the measurements? In order for me to detect if the problem is with our INA we need to make sure that you are measuring the expected shunt voltage.

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

I don't think that there is problem in the INA there is a problem in the board layout that I have made. Can you please go through the board layout and suggest changes that might have affected the output.

0 Mayrim Verdejo over 6 years ago in reply to Atharva Karaguppi

TI__Genius 15760 points

Atharva,

The input connections seems fine but I noticed you have an additional resistor connected, RC. This resistor is connected to a control voltage. What is this for? This might be the error, this might be changing your gain. Remove RC and try again. Let me know how it goes!

0 Mayrim Verdejo over 6 years ago in reply to Mayrim Verdejo

TI__Genius 15760 points

Hi Atharva,

Were you able to identify the problem?

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

Yes that resolved my issue.

However I have a new problem. I haven't changed my supply voltage but I am varying the load current from 0 A to 1 A. I saw that the expected values are coming up till 0.8 A ie corresponding to an output voltage of 1.6 V. But later as I increased the current from 0.8 A to 1 A the output would not increase. Basically the output voltage never exceeded 1.6 V. So I changed the value of the sense resistor to 0.05 ohm. This limited the output voltage to 1 V for variation of current from 0 A to 1 A. But still why is not going beyond 1.6 V.

0 Mayrim Verdejo over 6 years ago in reply to Atharva Karaguppi

TI__Genius 15760 points

Hi Atharva,

Did you change your circuit? With a 0.5 resistor, 0.8A load (around 400mV Vshunt) and RG = 49.9V (Gain of 10 V/V) you should be expecting around 4V on the output. Why are you expecting 1.6?

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

Sorry I forgot to mention I changed the sense resistor to 0.1 ohms. The supply voltage is still 12 V and the gain resistor is Rg = 100k now.

0 Mayrim Verdejo over 6 years ago in reply to Atharva Karaguppi

TI__Genius 15760 points

Atharva,

Thanks for clarification, calculations makes sense now. Based on the numbers you are providing me the device should not have any problem having 1.6V on the output. There might be something happening in your circuit. Did you tried replacing the LMP device for a new one? Maybe the device got bad with your old configuration. You were applying voltage in the RG pin, that is not recommended.

0 Atharva Karaguppi over 6 years ago in reply to Mayrim Verdejo

Prodigy 140 points

Ok I'll try that.

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LMP8646: Inaccurate output voltage