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Author Topic: Re: Mixup with MC33269xx-xx on the schematic- YES, you have both regs on boad  (Read 949 times)
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I don’t know if anyone notices but I think there is a problem with the Arduino schematics with their power supply. I think I have run across a mix-up of part symbol with part name and it seems that it crosses all the Arduino boards (UNIO to the MEGA 2560). I have the MEGA 2560 so I can personally speak from that model when I try to describe what I am seeing. (From here on out, I will refer to the Arduino MEGA 2560 as just MEGA- just for clarification.) Also, for those who have been talking about where to put your C2 capacitor in regard to MC33269xx-x.x you might want to read this too.

First, I think IC1 and IC2, for all the schematics that I have run across, the regulator power ICs symbol are swapped with the part name on the Arduino schematics. I am surprised that no one has picked up on this or caught the swap. I am talking about the MC33269x-x.x which you can see from the data sheet here >> http://www.onsemi.com/pub_link/Collateral/MC33269-D.PDF
Grumpy_Mike said in one of his post there is only one regulator on the board, I think there are both regulators on the board. I think who ever did the drafting for the power supplies were not paying attention to how they were drafting.

Second, if you go looking for the MC33269ST-5.0T3, you will not find it at any part house. Both Mouser and DigiKey do not have that part in 5 Volts. YES, the ‘ST’ part is suppose to be in a SOT-233 package, but if you look at that data sheet the ‘ST’ part only comes in the 3 Volt flavor. That does not change the fact that there is a MC33269 5 Volt SOT-233 package on your Arduino board (at least it is for the MEGA). Now here comes the part ‘B’ of my confusion- on any of the Arduino schematic I have come across, they show this part as a part with more than 4 pins. That would make this part a SO-8 part not a SOT-223 part. And the MC33269ST-5.0T3 is clearly a SOT-223 part, which would mean that this is IC2 on all our schematics.

NOW where in the heck you ask is this SOT-233 part on your Arduino board you ask? Well for the MEGA board; if you hold the board like this >> http://www.hobbyengineering.com/pics/i4312-ProductFront-400x223.jpg (thanks hobbyengineering.com for your picture- Sparkfun at this time does not have their pictures updated- they still have the old FTDI chip picture showing) it is north of the power jack soldered to the right side of the silver square (which is being used as a heat sink for the MC33269- just north of the power jack for the regulator). Also, you might have not notice that’s missing from all our schematics is the really cool poly fuse that’s between the SOT-233 package and the USB connector. I really wish Arduino would update their schematics- because the use of the poly fuse is so cool to use for current protection. Yes, I have read about it, just not seen any datasheets on which one they are using. And just like a good bone, the Lab loves to collect datasheets on cool things.

Now you’re asking where in the heck does the Blacklab think he’s sniffing the second MC33269 chip? Well if you look at that JPG picture that I gave you, I will walk you were it is at. Now look at the USB connector, look right of it and you can see the NEW ATmega8U2 that replaces the old FTDI chip for USB communications. South of that ATmega8U2 chip you will see a silver oval which is the crystal for the ATmega8U2 chip. And south of that is an 8 pin SOT-233 chip, which if you get your magnifying jeweler glasses out you will see it connect as you see IC1 in the Arduino schematic(s). Therefore IC1 is really the MC33269D-5.0 (NOT IC2). If you look at the two silver cans right of the Power jack those are the 74u capacitors. And the block just a little up and to the left of the two capacitors is the D1 M7 diode. Now, what I just noticed is where the C2 is placed on the 5 Volt side in the MEGA schematics and C2 is placed on the supply side in the Uno schematic { http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf }.

I hope I have proven that both power regulators are on board and it might give inspiration for you to copy their power design on the next board you make. Don’t forget to add that poly fuse to your next design too.
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please don't cross-post (post the same content in multiple threads.)
http://arduino.cc/forum/index.php/topic,52907.msg435153.html#msg435153
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Thanks for saying something. My problem is that this chip is being talked about in 15 different threads- same information being repeated over and over, along with some misinformation about usage of said chip. And there was at least three threads talking about support glue, which nobody pointed out that the way the support glue was being used, was not consistent. I am for updates (the next greatest Arduino), but it seems the updates are not educational to someone new to this environment (why you do X). What I mean by that is I read one place a capacitor needs to be place in once place, then the next thread said no it needs to be in another location. I understand rules change, but the example of the different boards we are given and their schematic for these boards, this capacitor is moving all over the place around the MC33269D-5. And for the record it’s the same value and type of cap. Yes, around 74HCxxx and 74LSxxx you place a 10mF or a 1mF cap. There is a reason you do that. But what I am physical seeing and what I am reading through the different threads, it does not add up.
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So did you have a specific question ?

The arduino hardware is relatively undocumented.  This is because (probably) it's not intended to be used as an education in hardware design, and it's mostly pretty trivial.  To really analyze it, you should note:

  • Don't assume that part numbering is consistent from design to design (Uno vs Uno Mega, or even Uno vs Duemilanove
  • Looking at the actual EAGLE documents is to be preferred over PDF files.
  • if you have a question, say exactly which document and/or board and/or posting you are looking at.
  • WRT decoupling capacitors, note that there are quite a few of them.  Whether the cap next to the regulator in the schematic is actually next to the regulator on the board is a somewhat academic, if they're the same value.

For example, the dual regulator layout issue is shown using EAGLE here:

The two chips occupy the same place; they can not both be on the board at the same time.
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