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UCC28951: UCC28951 Error-Amp Common-mode Input Voltage Range

Jeff Melvin
Prodigy 230 points

Part Number: UCC28951

On page 7 of the datasheet, under "VICM Common-mode input voltage range" it states "VICM range ensures parameters, the functionality ensured for 3.6V<VICM<VREF+0.4V and -0.4V<VICM<0.5V".

To me that means the error amp will function over a common-mode input range of -0.4...5.3V, although outside the 0.5...3.6V range it may not meet the other listed specifications.  Is this correct?

I am having issues with the error-amp output, COMP, railing at 4.283V (as expected) with inputs of EA+=5.015V and EA-=5.080V, while VREF=5.002V.  This was unexpected.

I would like to know what is meant by "VICM range ensures parameters, the functionality ensured for 3.6V<VICM<VREF+0.4V and -0.4V<VICM<0.5V".  Is there any possibility of output inversion or some other unexpected behavior outside the "ensured paraments" range of 0.5...3.6V?

  • +1
    TI__Intellectual 1170 points

    Hi Jeff Melvin,

    The range means, the allowed voltage at each pins. To ensure functionality, the EA- pin voltage should always in the range of  -0.4V<VICM<0.5V. Similarly, EA+ pin voltage should be in the range of 3.6V<VICM<VREF+0.4V. This is not matching with your test result. Based on your test result, EA+ and EA- pin are out of allowed range (5.015V and 5.080V). I suggest you to have a look at the schematic and make sure the connections are made as per recommendations in datasheet.

    Regards,
    Deepak K

  • +1
    Prodigy 230 points

    That's an interesting response.  If EA- is (too) low and EA+ is (too) high, then COMP will be forced high and the phase will be forced to maximum. 

    Obviously, the loop around the error-amp will force the inputs to be the same voltage.  The question is whether this is guaranteed over the full -0.4...5.3V extended input common-mode range.  Please advise...