How to deal with strobe and balance pins of this comparator for single supply operation?

Question

I want to use this LM311 comparator with a single supply. But these are pins called balance and strobe which confuses me. What are these for?

In a forum I read someone suggests pin 5 and pin 6 shorted for single supply operation. Should I also ground them? But the data-sheet says: "Do not connect strobe pin directly to ground, because the output is turned off whenever current is pulled from the strobe pin"

These pins should not be kept floating, but how should they be connected in case of single supply operation as a comparator or schmit trigger?

Answer 1

I don't believe this question has been answered properly yet.

Strobe

The Strobe Pin turns the output off. When the strobe pin is pulled to ground, a current of a few milliamps flows out of the device and the output turns off. So even if the inverting input voltage is higher than the non-inverting input voltage, is strobe is asserted, the output remains 'high'.

Balance

The Balance pin is used to help reduce the comparator's input offset voltage. Ideal comparators switch whenever one input's voltage is higher than the other. If the inverting input voltage is 3.000000V, and the non-inverting input voltage is 3.000001V, an ideal comparator will output 'high'.

However, non-ideal comparators may have a mis-match, where there is an offset voltage. You can imagine this as an ideal voltage source in series with one of the comparator's inputs. See the example below.

Below is a schematic modeling a (mostly) ideal open-collector comparator.

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Once the non-inverting input voltage crosses the inverting input voltage, the output switches.

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This happens the instant the non-inverting input voltage is larger, here at 3V + epsilon, where epsilon->0+

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Now if the comparator is modeled with a 5mV input offset voltage,

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The output doesn't switch at 3V, it switches at 3V - 5mV (2.995V).

The balance pin is to help the user trim the input offset voltage (reduce it ideally to 0V).

What to do with pins

Do not directly ground the strobe pin, if you want to use it control it with a switch (BJT) as shown in the LM311 datasheet (fig 18).

You can tie the balance pin to the strobe pin and leave them both unconnected, or you can just leave them both unconnected (floating). You can also use the balance pin and a potentiometer to trim the input offset voltage (fig 17).

Answer 2

The LM311 has lots of gain, and an output stage with massive transient currents that cause upsets to GND and to rails and to input pins.

You'll likely have oscillation. I suggest you reconsider your choice.

Answer 3

The datasheet says in figure 17:

If offset balancing is not used, the BALANCE and BAL/STRB pins must be unconnected. It is also acceptable to short pins together.