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Current monitor question #1: magnitude

Kevin Seaton
Prodigy 140 points
Other Parts Discussed in Thread: UCD7242, UCD9240

I'm using the UCD9240 to interface with the UCD7242 drivers.  My max output current on one output is only 1.2A with a defined fault current level of 1.56A.  If I let Power Designer choose the CS pin external resistance, it chooses a 10kohm resistor.  Since the current drive for this resistor is from the UCD7242 at 20uA/A, this would yield 2.0V on the CS pin with a 10A load.  I see that 2V happens to be the max input dynamic range for the CS ADC.  Given that my fault level is so much lower than that (1.56A), wouldn't it be beneficial to increase the sense resistor to something like 63.4kohm to improve ADC resolution at these lower currents?  Is there any danger in doing so, like over-voltage the CS pin input if the current goes above 1.56A for some reason?  I see that this pin can handle up to 3.8V (abs max).  But, since it's driven by a current source from the UCD7242, is it even possible to over-voltage this net?  Thanks.

  • 0
    TI__Genius 13700 points

    The UCD92xx device register that stores the converted CS ADC reading for the UCD7242's output current sets the maximum resolution obtainable to 15.625mA.

    The Fusion GUI recommends a 10K ohm resistor for the UCD7242's IMON pin (based on the 20uA/A of output load current) which feeds the CS input on the controller.  This will give a full scale reading of 12.5A at the 2.5V measurement limit of the 12-bit ADC (the 2.0V limit on the CS pin relates to the maximum DAC value that can be configured for the FAST OC comparator).  Dividing the full scale current reading (12.5A) by 212 (4096) will provide the measurement resolution at the 12-bit ADC which is 3.05mA per bit.  The measurement resolution is well beyond the resolution of the output current storage register.

    Thus any further increase in the IMON resistor value will not provide additional resolution to the monitored current value.