I am using the LM139 comparator chip. Now, I understand that the output of the comparators,or for that matter op amps are open drain/collector. SO they can only sink current and not source it per se.

I had a glimps of the data sheet which says Iout = 20mAmps. Is it the max current that it can sink ?

The datasheet screen shot is -Iout rating.

Also, in the LT1716, the output pin is stated as - "The output stage of the LT1716 can drive loads connected to a supply more positive than the device, the same as comparators with open collector output stages.".

Does this mean it (output load) has to be connected to a different voltage rail. Suppose I connect the comp to 44V (Vcc), then effectively the Load should be connected to a voltage rail > 44V.


Are my assumptions right ?

  • \$\begingroup\$ Could you change the second datasheet excerpt from picture to text? Text is searchable, unlike screenshots. \$\endgroup\$ Nov 19, 2015 at 6:49
  • \$\begingroup\$ I never pull up a LM339 etc open collector output above its supply rail .That doesnt mean you cant but if you can on your beetle from your component supplier or your junkbox can you say that it can be done on all beetles from all manufacturers ? I bet you that you cant because there are so many manufacturers . \$\endgroup\$
    – Autistic
    Nov 19, 2015 at 9:19
  • 2
    \$\begingroup\$ Not all comparators are open-drain. And no opamps are! \$\endgroup\$ Nov 19, 2015 at 10:52
  • \$\begingroup\$ @NickJohnson A few years back, I have seen an OpAmp with an open-collector output. The idea was to connect outputs of multiple OpAmps together such that the one with the lowest output "wins". The OpAmp was intended for power supply control loop. (I can't seem to recall the model, unfortunately.) \$\endgroup\$ Nov 20, 2015 at 1:02
  • \$\begingroup\$ @NickAlexeev I stand corrected. Still, pretty niche! \$\endgroup\$ Nov 20, 2015 at 7:59

2 Answers 2


If you don't care if it works and about reliability, then you are correct. In general you should never use 'absolute maximum' ratings for designing limits of normal operation- only to get an idea whether the device would survive a brief abuse of that magnitude.

A more conservative number is the current at which the maximum output 'low' voltage is guaranteed- 4mA. A prudent designer would derate that for the fact that the guarantee only holds for 25 degrees C - it would typically be worse at low temperature, so maybe 3mA for 500mV max drop.

enter image description here

Your interpretation of the Linear datasheet is correct- the load can be connected from the output to a voltage rail that is higher (or lower) than the chip supply voltage.

  • \$\begingroup\$ Did u get this screen shot from the LM139 datasheet please ? So, as long as the comp is not in its max rated voltage I can connect the comp Vcc to the load Vcc and all should be fine. \$\endgroup\$
    – Board-Man
    Nov 19, 2015 at 6:51
  • \$\begingroup\$ Got the data sheet. Then what is 20mAmps I out ?? \$\endgroup\$
    – Board-Man
    Nov 19, 2015 at 6:56
  • 1
    \$\begingroup\$ Briefly drawing up to 20mA will not immediately destroy the chip. There is no guarantee of maximum output voltage and it may eventually cause permanent damage. See note 1 in your screen grab. \$\endgroup\$ Nov 19, 2015 at 6:59

When being driven from an open collector output, the +ve terminal of the load has to go to a supply more positive than the driver ground.

When the o/c driver turns on, almost all of this voltage will appear across your load, the load supply voltage less the driver saturation voltage.

You can choose the comparator supply voltage, and the load supply voltage, independently, as long as you observe the recommended maximum voltages for each. This means you can power the comparator from any convenient voltage, while choosing the 'right' voltage for the load. The comparator supply voltage can be higher, lower, or the same as the load supply voltage. Usually, it's most convenient to power them both from the same supply.

Notice how the absolute maximum figures for voltage and current are larger than the typical or guarranteed figures. This is called stand-off, and is for your protection. While the chip should survive 20mA on the output, it is guarranteed to deliver 6mA while meeting the output voltage spec, and will typically manage 16mA. Similarly, there's a difference between the voltage specifications.

To use a driving analogy ... 6mA is how fast you'd take a corner with your mother in the car, 16mA is with your mates in the car, and 20mA is when you're being chased by the cops.


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