I'm working on a PT2399 project, the layout is pretty dense. I was looking over the schematic and it seems to me that I can get rid of C24, C25, C36, and C37 since they are tied to the +5V net and C5 and C8 are already handling the filtering. I just wanted to get some other opinions, in case I'm missing something. Any help would be much appreciated 8)
Looks like the 10u caps are reservoirs for the PT2399s, while the 100n caps are decouplers for noise. These are not much of an RC filter unless you add a resistor inline with the 5V bus, but you don't really need that, your incoming +9V is filtered OK. The 5V regulator will pass along whatever noise it receives, but it doesn't add noise.
The Princeton data sheet shows a 100n and a 100u on Pin 1 of a single PT2399. Ideally, decouplers would be right against pin 1 of each PT2399, so I'd be tempted to replace C5 with a 100u somewhere on the 5V bus, keep C8/C25/C37 (ensuring they're right at pin 1 for each IC) and drop C24/C36. You save 2 caps and actually get a better reservoir, and for a 5V bus you can find a skinny 100u 16V cap. The three 100n should be MLCC which are quite small.
Thanks! That's exactly what I was wondering. If you don't have the caps right next to pin 1, there are just 3 sets of both caps connected to the 5V power trace, which seemed redundant.
Quote from: mauman on March 21, 2022, 12:43:49 AM
Looks like the 10u caps are reservoirs for the PT2399s, while the 100n caps are decouplers for noise. These are not much of an RC filter unless you add a resistor inline with the 5V bus, but you don't really need that, your incoming +9V is filtered OK. The 5V regulator will pass along whatever noise it receives, but it doesn't add noise.
The Princeton data sheet shows a 100n and a 100u on Pin 1 of a single PT2399. Ideally, decouplers would be right against pin 1 of each PT2399, so I'd be tempted to replace C5 with a 100u somewhere on the 5V bus, keep C8/C25/C37 (ensuring they're right at pin 1 for each IC) and drop C24/C36. You save 2 caps and actually get a better reservoir, and for a 5V bus you can find a skinny 100u 16V cap. The three 100n should be MLCC which are quite small.
I agree as well. For a tight layout you can consolidate the electros into one and just add 100n decouplers for all three. It'll work fine.
Thanks, Brian! 8)