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TPS2420: Offset current

Jejomar Ildefonso
Genius 9490 points
Part Number: TPS2420

Hi Team,

Need your help on this. Our customer is measuring "offset current" in the 1μA to 2.5μA range in IMON. The measurements are done under load and no load conditions. The load conditions were from 1mA through to 1A, and the output was linear, however there is an offset.The offset current for IMON given on its datasheet is apparently a sinking current (due to the negative values).

He added that the offset can be cancelled through software. We just want to verify if this is indeed the offset current of the TPS2420 and not an abnormal behavior. Thank you for the help.


Kind Regards,

Jejomar

  • 0
    TI__Guru 57737 points

    Hi Lidefonso,

    I will look into this and get back to you today

  • 0
    TI__Guru 57737 points

    Hi Jejomar,

    Can you let us know what is the offset current measured when there is no load ? Also, how did customer measure this value and what is the accuracy of the measuring equipment ?

    How did customer calculate offset current when there is load ? Have they considered  variation in ILOAD/IIMON ratio as mentioned in the datasheet ? When there is load, the deviation in measured value from typical value can be due to offset current, part-to-part, temperature and process variations. 

  • 0 in reply to Praveen GD
    Prodigy 20 points

    Hi Praveen,

    I'm using five of the TPS2420 devices on my board. Four of there are being used to switch 12V loads, and have a 154k resistor to ground on the IMON output. The 5th is being used to switch a 5V load, and is using a 66k5 resistor to ground on the IMON output.

    On the 4 devices used to switch 12V loads, with no load applied but with the output enabled, I'm measuring 184mV, 416mV, 304mV, 216mV at the IMON pin respectively. This implies a current of between 1.2uA and 2.7uA (154k resistor).

    The device used for switching the 5V load is measuring approx 112mV at the IMON pin - giving a current of approx 1.7uA (66k5 resistor).

    The resistors are 1% tolerance.

    This offset current does not reduce with load applied - it increases linearly and I'm fairly sure we can zero it in software but I'd like to confirm that it is indeed an offset current and that we are correct to subtract the no load IMON voltage from our reading to obtain the actual voltage due to the load and therefore the load current.

    I tested this with the EVM board as well, and we were seeing similar numbers when we substituted in our resistors.

    Cheers,

    Dean

  • 0 in reply to Dean Claxton
    TI__Guru 57737 points

    Hi Dean,

    Thanks for your response.

    The current flowing into the IMON pin when there is no load current can be considered as offset current of the IMON pin.

    As you can see in the datasheet spec, the offset current can vary from 0uA to 10uA. 

    As your measurement shows the value is different for different ICs, are you going to cancel the offset  by measuring it and using the value in software for each IC separately?  Please be aware that this parameter can vary with voltage, temperature and also have part-to-part variation.

  • 0 in reply to Praveen GD
    Prodigy 20 points

    Thanks,

    I don't understand that the current is flowing into the pin though - its connected to ground - does the TPS2420 create a negative voltage internally? This current does not decrease as loaded, but increases and the resultant voltage is positive in relation to ground.

    We would be zeroing the current on each device independently with no load applied as part of a calibration setup. Will keep in mind the variation with temperature and load - we can test this and see if its worth correcting for (the reading may be close enough for this application once the zero load current is zeroed).

    Cheers,

    Dean

  • 0 in reply to Dean Claxton
    TI__Guru 57737 points

    Hi Dean,

    The internal circuit as per design sinks current when there is no load (i.e. offset Current). As the load current increases the IMON pin sources current as per  ILOAD/IIMON ratio because of which you see positive voltage across IMON to GND when load increases.

    I agree with your approach of calibrating the offset error. 

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Dean,

    Hope your query is addressed. In case you do not have further questions, can we close this thread ?