TPS24720: Imon output voltage is off

William Sung36

Part Number: TPS24720
Other Parts Discussed in Thread: LM321

Hi,

I am using a TPS24720 as a efuse for 12V, 10.9A. Using the spreadsheet tool for this part, i have come up with the following values:

Rsense = 2mohm

Rset = 44.2ohm

Rimon = 1.37Kohm

One of the useful features of the TPS24720 is the Imon output which i am feeding into a LM321 op amp with a gain of 4.01 which is then feed into a ADC to monitor the current that is being passed through the TPS24720. What i am seeing now is that the measured voltage on the Imon pin does not match my calculations of what the current should be. In the lab i have tested with the following conditions:

Vin = 12V

I = 2.47A

Vimon = 183mV (measured)

Based on the selected resistors, i get a calculated value of 153mV for Vimon. I repeated this test with the op amp disconnected from Imon and i get measure the same Vimon value. What am i doing incorrectly here as the measured and the calculated value is off by a lot.

Thanks

over 5 years ago

0 Praveen GD over 5 years ago

TI__Guru 57737 points

HI William,

Can you please share your schematic and Design Calculator that you used for your your design ?

0 William Sung36 over 5 years ago in reply to Praveen GD

Intellectual 395 points

Hi, please see attached picture of the schematic and the excel file that was used.

Thanks

WilliamTPS24720_Design_Calculator_REV_F KU040.xlsx

0 William Sung36 over 5 years ago in reply to William Sung36

Intellectual 395 points

Picture of schematic is attached.

0 Praveen GD over 5 years ago in reply to William Sung36

TI__Guru 57737 points

Hi William,

Your Schematic looks fine. As you said, for a load current of 2.47A, the IMON voltage should be 153mV. There shouldn't be so much deviation in IMON Voltage.

The IMON accuracy depends on

Load Current measurement accuracy
Rsense, Rset and Rimon accuracy
Offset Voltage of the Internal Amplifier - 1.5mV, which can cause significant error at low Load Currents.

Can you please check the above points on your board ?

0 Praveen GD over 5 years ago in reply to Praveen GD

TI__Guru 57737 points

Hi William,

Can you update us on your findings?

0 William Sung36 over 5 years ago in reply to Praveen GD

Intellectual 395 points

Hi,

I went back and verified my load, voltmeters, resistors and such and all that stuff looks good. I am now performing my measurements without the op-amp and just measuring the voltage at the Imon pin. What I find interesting is that it looks like there is a offset to this measurement. If I increase the current draw by 1A, the measured voltage is a reflection of that. For example when my system is idling my power supply will indicate a current draw of 150mA but my measured current on Imon is 30.8mV which is 50mA. When add a 1A load, my power supply indicates a power draw of 1.15A and I would measure on Imon 97mV which is 1.56A. The load difference is 1A which is reflected in the measurement but the absolute value is not correct. Is there a offset that needs to be adjusted some how?

Also, the measurement of Imon, is it only goverened by Rset, Rsesne and Rimon or is there something else that effects this measurement?

Thanks

William

0 Praveen GD over 5 years ago in reply to William Sung36

TI__Guru 57737 points

Hi William,

Yes, as I mentioned before there is effect of offset of internal amplifier on the current measurement. As you know the effect of Offset is higher at low voltages, this is the reason why you see more error in current measurement at low currents which correspond to low sense voltage.

Vimon = ((Iload * Rsense)* Rimon)/Rset

Vimon = (Vsense* Rimon)/Rset

Considering error due to offset voltage,

Vimon = ((Vsense +/- Voffset)* Rimon)/Rset

So, the effect of Voffset reduces as the Vsense increases. You should see less error at high Load Currents.

As there is a range for offset (-1.5mv to +1.5mV), it can be difficult to calibrate for the offset error.

0 William Sung36 over 5 years ago in reply to Praveen GD

Intellectual 395 points

Hi, that makes more sense now, so I see why I will always get this offset.

Is this offset voltage constant across the load range?

Thanks

0 William Sung36 over 5 years ago in reply to William Sung36

Intellectual 395 points

Another quick question on this part, if I want to use it for a much higher current application, like around 40A, 12V, can I parallel up 2x the Rsense to help with power dissation across this resistor?

0 Praveen GD over 5 years ago in reply to William Sung36

TI__Guru 57737 points

Hi William,

Unfortunately, we can only guarantee the range within which the Offset voltage can vary, i.e. -1.5mV to +1.5mV. It will be difficult to provide a constant offset value as they vary across process, operating condition and temperature.
Yes you can parallel Sense resistors and mosfets for high current applications. But, you need to take proper care in your layout to avoid problems. For Sense resistors, make sure you follow kelvin connections.
You can get more information here - e2e.ti.com/.../choosing-the-right-sense-resistor-layout

0 Praveen GD over 5 years ago in reply to Praveen GD

TI__Guru 57737 points

Hi William,

You can consider the below layout if you want to use 2 sense resistors.

Do you have any other questions? If not, can we close this thread ?

0 William Sung36 over 5 years ago in reply to Praveen GD

Intellectual 395 points

Thanks for the suggestion.

Power management

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TPS24720: Imon output voltage is off