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LM119QML-SP: Use Zener on V+ supply input

Mark Guastaferro
Genius 11205 points
Part Number: LM119QML-SP
Other Parts Discussed in Thread: LM139-SP

Customer would like to use LM119-SP but spec indicates that the positive power supply input should not be more than 16V above ground pin voltage.  In their case, they have a 23.5V power supply and V- will be grounded. 

Can they place a 10V Zener in series between the positive power source and V+ pin to drop the comparator voltage down to an acceptable voltage?  Any concerns to look out for? 

Note: They would normally use the LM139-SP but long lead time issues are forcing an alternate approach. 

  • 0
    TI__Guru 52046 points

    Hello Mark,

    The 16V limitation above ground has to do with the output stage and the way it internally level-shifts.

    Yes, they can add a Zener in series, with an additional resistor to ground to ensure that there is always some minimum current flowing through the diode.

    Just be sure to add full bypassing at the V+ pin, as now the supply is "soft" and separated from the supply bulk bypassing. There is a change of supply current of almost 1mA when the output toggles, so the V+ pin needs to have a low AC impedance. So depending on the toggle rate, I would recommend parallel 10uF and 0.1uF bypass caps to V+ to prevent oscillations.

    The inputs still need to be at least 2V below V+, so make sure that their expected input range does not violate the input range (for example, +10V limit for a +12V upper supply).