LM311: Input voltage on IN+ pin

The absolute maximum ratings forbid +24 V. The input voltage must be no more than 15 V above the midpoint between the supplies (see V_I), and the magnitude of the input voltage must never exceed the magnitude of the supply voltage (see footnote 4).

If possible, change the circuit so that the comparator is supplied by the 0 V and 24 V rails.

Otherwise, I'd recommend a diode clamp to the positive supply, and a current-limiting resistor in front of that.

Hi Mesh,

in order to provide full input protection, especially when the input signal comes from outside and can be applied during power-down of LM311, I would do it this way:

mesh_lm311.TSC

D1 and D2 clamp the input voltage to the positive supply voltage of LM311 and to signal ground. R2 limits the current through these diodes. At the same time R2, R3 and R4 form a voltage divider which limits the input voltage at "node 8" to +14.4V. Because of this, D1 is not turned-on during normal operation. This is important because the supply voltage may not be able to accept reverse currents flowing through D1. This voltage divider also limits the voltage at the positive supply voltage pin of LM311 to below +14.4V during power-down when +24V is applied to the input.

Because the input voltage swing from 0V to +24V is decreased to 0V to +4.8V by the action of this voltage divider, the reference voltage is changed to +2.4V accordingly by the jhelp of R5 and R6. C3 helps to not only reject supply voltage noise but also provides a very effective suppression of unwanted toggling glitches being injected from the output to the -input via input to output stray capacitances.

The next simulation shows what happens during power-up or power-down, if the +24V input signal is already applied:

mesh_lm311_1.TSC

The input voltages at the +input and -input of LM311 are never exceeding the positive supply voltage of LM311 by more than 0.24V, because R3 and R4 divide down the voltage drop across D1 by a ratio of 3:1. Limiting the overvoltages at the inputs to below 0.3V very effectively prevents damage and latch-up.

Of course, some input low pass filtering can still be added -if wished- by the help of a cap in parallel to D2.

Kai

Thank you, Kai and Clemens, these are all great explanations and inputs. 

Mesh,

The diode and resistor divider scheme Kai talks about is a great way to protect the comparator when you have large inputs above the rails.Let us know if you have any more questions.