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LT1716 output impedance

Styx

Active Member
I am presently trying to debug a comparator circuit someone else has designed and it is presently built around an LT1716 comparator
originally the threshold were miles off the design case and via LTspice the circuit now toggles at the desired points (~3.9 , ~3.6). The problem is, this is based upon noting the output voltage was 7.9V (as oppose to the 15V - 210mV). This part has a low output current source and thus this will be responsible for the reduction in output voltage and the change in thresholds.

What I am struggling with is how to determine the output voltage analytically rather than via simulation. I have opened up the SPICE library which comes with ltspice in an attempt to determine the equivalent output impedance but I am struggling. Presently everything aligns when the output of the LT1716 is assumed to be ~47k


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Any help would be appreciated
 

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Styx

Active Member
while most comparators are open collector, this one isn't, it has a class-B output. The long term solution is to replace this comparator with a more suitable one but presently it is justifying the anticipated variations.

I did look at including a pull-up resistor and while this does permit the output voltage to be raised, it is still influenced by the output capability of this comparator so it doesn't fully mitigate the variation, hence the want/need to understand it.
 

alec_t

Well-Known Member
Most Helpful Member
With the circuit as you show it you are relying on the weak pull-up current source in the IC. The datasheet says that is a minimum 60uA and typically 85uA. That is not going to give you predictable switching thresholds. Methinks you need an external pull-up which over-rides the internal one.
 

crutschow

Well-Known Member
Most Helpful Member
The "output impedance" of most comparator circuits has no single value as it varies significantly between a high and low output.
Below is the output drive capability of the LT1716.
As noted by alec_t, the source current is very low, thus it essentially acts as an open collector output, so you should add a pull-up resistor to give a solid high output (staying within the 10mA output minimum current when the output is low).

1593523929654.png
 

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