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LM2903B-Q1: Common Mode Voltage change over temperature

Chris D
Prodigy 40 points
Part Number: LM2903B-Q1
Other Parts Discussed in Thread: LM2903B, LM393LV

Tool/software:

Hi Team,

I have a design which uses the LM2903B comparator. It runs on Vcc=4.2V. One of the inputs is fixed at 2.3V, while the other varies between 0.3V and 3.9V.

The datasheet states the VCM at ambient is Vcc-1.5 ( = 2.7V) so everything should work fine (at least one input is within VCM).

However, across temperature the VCM reduces to Vcc-2 ( = 2.2V), so the comparator may not work in this situation.

App Note SNOAA5F (https://www.ti.com/lit/an/snoaa35f/snoaa35f.pdf?ts=1748330538078) page 17 seems to suggest the VCM gets worse as temperature decreases.

"The input range from 0 to 3.5V (VCC - 1.5V) is valid at 25°C and above." ... "This results in the input voltage range changing at -4.2mV/°C over temperature (note the negative sign!), necessitating the VCC - 2V over temperature specification."

Am I understanding this correctly? At temperatures between +25C and +125C, I can assume the VCM is Vcc-1.5? And temperatures below +25C, I can derate the VCM at -4.2mV/C? My device needs to operate up to 95C so, if true, this would solve my issue.

As a side note, if I derate at -4.2mV/C from +25C down to -40C, the VCM should reduce by  65*4.2 = 273mV. This should mean the VCM across temperature is Vcc-1.773, not Vcc-2. Is the extra allowance just safety margin, or is there some additional factor that I'm missing.

Best Regards,

Chris

  • 0
    Prodigy 40 points

    And yes, I know I should use the LM393LV instead. I have swapped this in for the next revision of design but I have units built and need to know the behaviour of them with an LM2903B.

  • +1
    Guru 318700 points

    Yes, the range gets worse at cold temperatures.

    The datasheet guarantees the VCC − 2 V and VCC − 1.5 V limits; all other values mentioned in the appnote are typical.

  • 0
    TI__Guru 63416 points

    Hi Chris,

    Yes. The VCM range shrinks at cold. Your circuit will work on the bench at room temp, but could fail at cold. Going hot actually helps you.

    At 1.773V you are on the edge of failure at cold - and that does not take into account device process variations. Add ±30% on top of that...

    VCM is not a tested or trimmed spec so it has very wide variation.

    The VCC-2V is conservative and ensures that the device will firmly meet datasheet specifications (Vos, Ib, Icc, etc).

    As mentioned in the appnote, the one-input-within-range "feature" is not guaranteed. It is a nice-to-have, but it is not specified behavior.