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LM3881: EN pin threshold MIN...MAX

Steve Mowbray
Prodigy 40 points
Part Number: LM3881

Tool/software:

Hi 

LM3881 enable circuit is described as an internal comparator with precision reference; however in Section 6.5 Electrical Characteristics a threshold precision of 500mV seems to be implied which isn't particularly precise

Section 6.6 Typical Characteristics is the precision we require but if the unit to unit variation can be 1.0V...1.5V around 1.22V then the LM3881 doesn't provide a precise solution to our requirement...

So is the EN threshold range of 500mV real or am I misunderstanding the table?

Thanks

Steve

  • 0
    TI__Guru 66000 points

    Hi

    500mV range cross temp is real for the EN. 

    what's your EN threshold defined ?

    Regards

    Yihe 

  • 0 in reply to yihe
    Prodigy 40 points

    Hi Yihe

    Figure 3 shows 30mV range across temperature but Section 6.5 Electrical characteristics implies 500mV unit to unit variation around typical -- if unit to unit variation is as stated in datasheet then the built in precision comparator is not suited for our design (we need <40mv precision across temperature and unit to unit).

    500mV of precision around 1.22V doesn't appear to meet the definition of precise to me hence my confusion and this technical question....

    Thanks

    Steve

  • 0 in reply to Steve Mowbray
    TI__Guru 66000 points

    Hi Steve

    Figure 3 is show a single device and Section 6.5 is a full Qual cross different wafer lot, units, temp. 

    Regards

    Yihe 

  • 0 in reply to yihe
    Prodigy 40 points

    Hi Yihe

    Yes that is my understanding also; however I am surprised to the point of disbelief by the 500mV (eg 1.22V +/-20%) datasheet stated unit to unit variation for a "precision" comparator -- am I missing something in my understanding or is there a typo in the datasheet?

    Thanks

    Steve 

  • +1 in reply to Steve Mowbray
    TI__Guru 66000 points

    Hi Steve

     I understand your concerns. But most the time, the ENABLE signal is from external logic input which is very stable. or from a voltage divider of VCC which is also a reliable source.

    once the EN is above 1.5V,  the variation of the EN can be ignore since it is always above the threshold.

    Regards

    Yihe

  • 0 in reply to yihe
    Prodigy 40 points

    Hi Yihe

    Thanks -- so it seems the EN input is most useful as a logic input with Vih=1.5V -- an external precision comparator is required to detect VCC rising/falling threshold voltages to reliable 40mV range

    Regards

    Steve