How to find replacement chips

[pharaoh3]

The title is a bit misleading, so allow me to explain. It seems like chips/ICs come in many varieties even if they are in the same series. I don't understand why a chip has versions with slight variations in the model number or what these changes mean.

For example, a PCB I am working on has a SN74LS139N chip. Pasting this into Digi-Key gets me nothing. However, if I paste SN74LS139 (we removed the "N") a few different chips pop up. Many of these chips appear to be in the same series, but that does that necessarily mean they are compatible?

Another example is a chip labeled LH5168FB-10L. Pasting this into Digi-Key gets me nothing, but if I simply type LH516 then I a few SRAM units pop up that may work. Yet, like before, I cannot tell if these will work as a replacement for my current chip.

I would sum up my questions as so.

• How do the naming conventions of chips work?
• How can you tell if a chip is a suitable replacement?

[RoGeorge]

Search for its datasheet
https://duckduckgo.com/?q=SN74LS139N+datasheet

First result is this:
https://pdf1.alldatasheet.com/datasheet-pdf/view/5655/MOTOROLA/SN74LS139N.html

Read the PDF datasheet to find what the suffix means, here it means the chip is encapsulated in a plastic DIP14 capsule.

[ledtester]

As RoGoeorge mentioned, the suffix usually indicates the package and temperature rating of the chip.

According to this datasheet:

https://www.ti.com/lit/ds/symlink/sn74ls139a.pdf?ts=1652417000549

the "N" means a DIP package and an "A" means consumer temperature range (0 - 70C) rather than the military spec range (-55 to 125).

By the way, octopart.com is also a nice resource to use to find chip suppliers and datasheets.

[edavid]

As RoGoeorge mentioned, the suffix usually indicates the package and temperature rating of the chip.

According to this datasheet:

https://www.ti.com/lit/ds/symlink/sn74ls139a.pdf?ts=1652417000549

the "N" means a DIP package and an "A" means consumer temperature range (0 - 70C) rather than the military spec range (-55 to 125).

You've done a nice job of demonstrating that there is no standard.

For this part (and all TI TTL parts), the A suffix means an improved design.  The temperature range is indicated by the 54 vs. 74 prefix.

[ledtester]

You've done a nice job of demonstrating that there is no standard.

For this part, the A suffix means an improved design.  The temperature range is indicated by the 54 vs. 74 prefix.

You're right!  -- it's hard to notice the difference on those grainy PDF copies of 30-year old datasheets!

Well, anyway, the answer is to _carefully_ read the datasheet.

[edavid]

The title is a bit misleading, so allow me to explain. It seems like chips/ICs come in many varieties even if they are in the same series. I don't understand why a chip has versions with slight variations in the model number or what these changes mean.

For example, a PCB I am working on has a SN74LS139N chip. Pasting this into Digi-Key gets me nothing. However, if I paste SN74LS139 (we removed the "N") a few different chips pop up. Many of these chips appear to be in the same series, but that does that necessarily mean they are compatible?

Another example is a chip labeled LH5168FB-10L. Pasting this into Digi-Key gets me nothing, but if I simply type LH516 then I a few SRAM units pop up that may work. Yet, like before, I cannot tell if these will work as a replacement for my current chip.

I would sum up my questions as so.

• How do the naming conventions of chips work?
• How can you tell if a chip is a suitable replacement?

Unfortunately there is no real standard or convention.  If you are old enough, you will know the history of the parts.  Otherwise, asking here is a good way to get suggestions.

For the chips you mention:

SN74LS139N: The base part number is 74LS139, but you will have to manually screen for the DIP packaged parts.  An experienced person would also consider 74HCT139 as a substitute.

LH5168FB-10L: The base part number is 5168, but an experienced person would know that those Sharp parts weren't too popular outside Japan, and would be looking at more common substitutes like various flavors of 6264 and 5165.  Obviously you have to match the package and 100ns access time.  If there's battery backup, you also have to worry about standby current.

[tepalia02]

Suffix does not really matter that much. Two ICs with different suffixes are functionally the same. The slight differences are in power consumption, temperature, voltage ratings etc. 

[edavid]

Suffix does not really matter that much.

Not true if the suffix indicates a different package or access time!

[jmelson]

You're right!  -- it's hard to notice the difference on those grainy PDF copies of 30-year old datasheets!

Well, anyway, the answer is to _carefully_ read the datasheet.

UGH!  It is a catastrophe!  There are now so many package styles that it will drive you crazy. There is DIP, SOIC, SOP, SOW, SSOP, TSSOP and on and on.  One has to be VERY careful to check the body size and lead spacing to make sure you are getting the right package to fit the board layout.  Sometimes I get caught buying the wrong size part, very frustrating.  Now, when parts are scarce, you have to resist buying some stock you find at some distributor until you have verified the package.
Jon