LM555 vs. NE555: What Difference is between LM555 and NE555?

The letter prefixes in front of IC component numbers are frequently assumed to signify the IC's manufacturer (though this is not always the case) – although this is not always the case. While certain chips have a large number of second sources with the same part numbers and/or prefixes, there are typically minor variances between each of these components due to manufacturer optimizations and revisions. The  NE555 timer IC, originally from  Signetics, and the LM555, which was originally second sourced from  National Semiconductor, are two examples of this.

The following figures show the pinouts of LM555 and NE555:

LM555 Pinout

NE555 Pinout

The following circuits show the internal circuits of LM555 and NE555:

LM555 Internal Circuit

NE555 Internal Circuit

Because most second-source ICs are promoted as 'plug-in' replacements for the original, little is done to call attention to the fact that the IC's internals may have been altered slightly. Fortunately, manufacturers include a (simplified) schematic diagram with the datasheets, which we shall examine.

One of the most notable differences between the two ICs is the use of current mirror active loads in the LM555.

Despite being 'pin-compatible, the LM555's modifications cause the circuit to act very differently than the NE555.

We can see from the internal design that the LM55 uses active loads instead of fixed resistors. One of the most significant advantages of switching from resistors to active loads is that output timing is less dependent on temperature and supply voltage.

Another difference is that the trigger signal is prioritized in the original NE555, whereas the threshold signal is prioritized in the LM555. When pin 2 (trigger) is less than 1/3VCC, the output of a 555 timer is high, and when pin 6 (threshold) is greater than 2/3VCC, the output is low. The output of the NE555 is low if the trigger pin is low while the threshold pin is high. The output of the LM555, on the other hand, remains low even when the trigger pin is low, and only becomes high when the threshold pin is low.

Additional undesired pulses have been reported on the LM555's output, which could be related to the various priorities assigned to the threshold and trigger pins.

The following table shows the major differences between LM555 and NE555:

Because there may be slight differences and improvements between components from different manufacturers, believing that the identical part from a different manufacturer is totally 'pin-compatible' is sometimes incorrect. It is always a good idea to test chips before using them in a specific application.