Purpose of LM358D and NDT2955 on Arduino 2009

Other Hardware 3rd Party Boards
1

Hi there,

I am wondering if someone can explain to me the reasoning behind including this combination of a mosfet and op-amp on the on the power input side of Duemilanove board.

The intent appears to be to separate the USB 5V bus from the on-board power voltage regulator if external power is available and the on-board 5V Voltage Regulator is running. Is it a USB-spec that a USB device cannot backfeed 5V onto the USB Bus?

I am designing a board with two Arduino 328s, one operating at 5V, the other at 3.3V, and I would like to avoid using all these chips if it's not necessary. There is no USB connection, just FTDI and ICSP headers, as well as on-board voltage regulation via two LDO voltage regulators. Should I disconnect the FTDI and ICSP power supply pins via a device like the NDT2955 every time the on-board power supply is also supplying power?

Many thanks for any help!

2

Should I disconnect the FTDI and ICSP power supply pins via a device like the NDT2955 every time the on-board power supply is also supplying power?

Simply don't wire the USB +5vdc power from the connector/FTDI chip to your board, just the ground and signal lines. Then you can always use your external power source regardless if you are plugging into the FTDI signals or not. It's never a good engineering practice to hardwire two +5vdc sources together and that is the purpose of the arduino's auto-voltage switching circuit. On much older Arduino USB boards they just used a simple three pin selector clip to manual switch between USB power or +5vdc from the on-board +5vdc regulator, a simpler and to my mind more elegant solution.

Lefty

3

I "used a simple three pin selector clip to manual switch between USB power or +5vdc from the on-board +5vdc regulator, a simpler and to my mind more elegant solution" on my "Bobuino" too. So one can power up with the reset-enable deselected, and add/remove the USB connector at will.

http://www.crossroadsfencing.com/BobuinoRev17

4
  1. It is VERY RARELY a good idea to "back-feed" anything, whether there is a spec or not. And I'm sure there is a specific prohibition for USB.

I agree. Yet that is exactly what they are doing to the 5v regulator chip on the Arduino when power is being fed from the USB port.

@Constantin
You should take a look at the power configuration on the Modern Device Bare Bones Board. Use the BBB Rev.E instructions link here: http://shop.moderndevice.com/products/bbb-kit .

Don

5

Thanks so much for the insights. For the board I put together, I simply omitted any traces to/from 5VDC on the FTDI headers.

The boards came together nicely, the software is proven, now I just have to bake them in my new SMD toaster oven. Wish me luck!

6

If I wanted to power only from the AC adapter but still be able to program via USB -- I could remove the LM358, NDT2995 and FDN340P from the circuit correct? Everything else would remain in this schematic, right?

Screenshot_3_27_13_8_57_PM.jpg
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7

I would just remove R10.

It would probably be a good idea to put a jumper across R6 as well, effectively putting 0v on pin 3 of the IC.

Don

8

Mavromatis:
If I wanted to power only from the AC adapter but still be able to program via USB -- I could remove the LM358, NDT2995 and FDN340P from the circuit correct? Everything else would remain in this schematic, right?

http://arduino.cc/en/uploads/Main/arduino-mega-schematic.pdf

I would just remove the F1 thermofuse as that is probably the easiest component to deal with. Note that any given board will not have both T1 and T2 mosfets mounted, just one or the other.

Lefty