Inquiring as to whether the LM139A is suitable for 28V to 3.3V translation. What configuration would be ideal? I was thinking of single supply (28V). Vref of +12V, and pulling up the output to 3.3V. If I am interfacing with a SN74LVCH244A, how would I determine which value pull resistor I need. Thanks.
I want to ensure I understand the operation of the LM139A. If I have V- set to 12V, and V+ is either 0v or 28V, Vcc = 28V. Being that the output is open collector, once V+ is 28V. This pulls the output to GND. Once V+ is 0V, OUT can be pulled up to let's say 3.3V.
In other words, it's inverting. What if I put V- to 28V or 0V, and V+ to 28V. Would my output be GND, if V- is 0V?
The output of a comparator will go to the rail of whichever input is greater. If (IN+ > IN-) the output will go to the positive rail. If (IN- > IN+) the output will go to the negative rail.
Since the LM139 is open-drain when IN+ > IN- the output will go to the voltage that it is being pulled up to, in your case, +3.3V.
See attached figure and TINA simulation to get yourself going. When the input (VG1) is greater than 12V (IN- > IN+), the output is pulled low. When VG1 is less than 12V (IN+ > IN-) the output is pulled up to the pull-up voltage. Pull-up resistance will affect the rise time during low-to-high transistions and current dissipated in the low-side transistor inside the LM139, a 1k - 10k resistor will likely work fine in your application.
For your "What if" case, if V- = 28V and V+ = 28V (assuming this is within the common-mode input range of the device) then the output will go to the rail depending on the polarity of the input offset voltage of the comparator. Over temperature/time/humidity and many other environmental conditions the input offset will drift so your output may go to one rail or oscillate between the two.
What is the speed of your 0-28V signal?
Let me know if you have additional questions.
Regards,Collin WellsPrecision Analog
Regards,Collin WellsPrecision Linear Applications
Understand, so in my case IN- will be 12V, and IN+ will range from 0-28V. So when IN+ > IN-, the output will be pulled up to 3.3V. The 28V signal is a discrete signal, not sure of the rise time. Does the speed have an impact on the LM139As' ability to track?
As long as the speed of the signal does not exceed the switching capabilities of the comparator, the LM139 should be able to "Track" your input. Response time is listed 1.3us with a 5mV overdrive, so don't design the system to track signals that will transition close to that speed.
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