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LM139AQML-SP: Unused inputs

Mihir Gupta65
Intellectual 2155 points
Part Number: LM139AQML-SP
Other Parts Discussed in Thread: LM139AQML, LM324

Hi Team,

A customer of mine has a question about the LM139AQML-SP

In the Application Design Guidelines SNOAA35–April 2019 under chapter “5.1 Unused Comparator Connections”, they describe how to connect the unused comparators.

It requires resistors in the input, floating outputs and biasing the inputs.

 

However in our current designs, the connection of Unused Comparators is done as described in the datasheet LM139AQML - SNOSAH8G:

“All pins of any unused comparators should be tied to the negative supply.”

 

For us it is important to know if , with the current implementation, there is a risk. Finally needs to modify existing designs?

 

Our analysis shows, that:

-        Connecting the output to the negative supply is not a problem. Supply voltage is 15V.

-        Connecting the inputs to the negative supply without resistor should not be a problem. ? Pins are directly connected via short tracks to the negative supply.

-        Having no bias voltage, could not be answered by us. Is there a risk of an internal oscillation? Pins are directly connected via short tracks to the negative supply.

 

I see from this other thread that the LM139AQML-SP does not have internal ESD diodes, so input current limiting would be unnecessary.

https://e2e.ti.com/support/amplifiers/f/14/t/797303

Could you please clarify what the best practice here should be?

Regards

Mihir

 

  • 0
    TI__Guru 63396 points

    Hello Mihir,

    Section 5.1 in that appnote was done to be consistent with the LM324 appnote.

    Yes, the inputs can be grounded *IF* they are directly connected to the GND pin, where there is little or no chance for transients to cause a voltage difference between the inputs and GND. Problems could arise if, say, the GND pin is connected to the power GND, and the pins are connected to the signal ground with transients between the grounds.

    The small risk of shorting the input pins is that some devices with zero offset could drift through zero (caused by temperature or supply voltage changes) and cause the output to "chatter". Since this is a open-collector output, if there is no pull-up resistor, the output will not move. The only thing you would see is a tiny (a few microamps) variation in the supply current.

    Current limiting is still necessary. There is still a body "diode" on the input to GND, even though it is not a dedicated ESD device.