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TPS2420: IMON output when no current is flowing through the power switch

Part Number: TPS2420

Hello,

In testing the current monitoring of one instance the TPS2420 we are using I see IMON output of ~190mV when no current is flowing though the TPS2420. There are multiple of the parts in the design, all of them show IMON output at varied levels when no current is flowing through the device. (The TSP2420 is enabled, but no load is connected.) Is this expected behavior? I do not see a datasheet specification for the IMON current accuracy when Iout is 0A. 

I have attached the schematic page as well as a scope shot showing the IMON voltage over the 40.2K resistor going from no load to a load. The offsheet connection is to the ADC used to check the current.

What would cause a varied offset voltage on the IMON? It is messing up our current measurement. 

Thanks,

Steve

7 Replies

  • Hi Steve,

    Is the offset voltage observed just after the device is enabled? whether the offset voltage on the IMON pin dies out when the Vout reaches steady state. ?

    It is recommended to ignore IMON information until the device is fully enhanced.

    There will be offset current (-10uA to 0uA) at IMON when no current flows through the device. It is mentioned in the  Electrical characteristics table.

    Can you please share test waveform of IMON voltage along with Vout voltage.

    Thanks and Regards,

    Rakesh

  • In reply to Rakesh:

    Rakesh,

    The offset voltage is present before the load is present but after the device is enabled and the power is good. We are ignoring the IMON until the PG_N is low (active) as recommended in the datasheet. I have attached a scope shot showing the 12V Vout present for the time prior to and after the load was attached. In this case the load was a 300mA load. The IMON was ~190mV before the load and ~510mV after the load across a 88.7K Ohm IMON resistor. 

    Please let me know what other information would be helpful in understanding this behavior.

    Thanks,

    Steve

  • In reply to Steven Goerlich:

    The other interesting (and unfortunate) part to this behavior is that we have many (8+) of the TPS2420 parts in the design and the initial offsets are not consistent even though the design of each TSP2420 is essentially the same. (Some of the resistors are different for different fault thresholds.) The initial offset IMON voltages vary from tens of millivolts (30mv+) to hundreds of millivolts (200mv+.) This also means that we can not just calibrate the offset out of the system easily...
  • In reply to Steven Goerlich:

    Hi Steve,

    Thanks for the scope shots. I am also checking with the design team on inconsistency whether this is the expected behavior for this device at low currents. I will get back to you as soon as possible.

    Regards,
    Rakesh
  • In reply to Rakesh:

    Hi Steve,

    The current monitor accuracy is very wide for this device at lower load currents. For example at 500mA, the Iload to IMON gain variation is +/- 45%. 

    The offset IMON current variation is also wide up 10uA, which makes IMON offset voltage at no-load for 88.7K Ohm resistor too wide. 

    I would like to know the use case of current monitor for your application and what kind of accuracy is needed?

    In which phase is this design? If possible, please look at new device TPS25944 which has good current limit and current monitor accuracy at lower load currents. Please let me know how I may help to meet your requirement.

    Best Regards

    Rakesh

  • In reply to Rakesh:

    We were looking to enable / disable and measure current through the device. Normally less than 1A @ 12V. I expected a better accuracy than 50%, which also seems to be generous at no load. We have boards and will use them for now. I expect we will change the parts in the next build. Do you have any footprint compatible options that will preform better that we can test?

    Steve
  • In reply to Steven Goerlich:

    Hi Steve,

    Unfortunately we don't have other parts which are footprint compatible with TPS2420.
    Please let me know when you go for next build, I will suggest more appropriate device to fit your requirements.

    Best Regards,
    Rakesh