Member #275234

Member Since: December 13, 2011

Country: United States

  • A couple of points based on looking at your schematic:

    1) I would not use NPN BJTs to drive your high-side P channel FETs. If you are worried about drawing current from the Atmega2560 I/O pins, you should use N channel FETs in place of the NPN BJTs since N channel FETs do not sink any current at the gate in order to turn on. A good example of an N channel FET you could use for this would be the IRFML8244, at 3.3V on the gate the part is fully on. And you also can get rid of the 100 Ohm resistors if you use N channel FETs.

    2) Consider replacing the U3 and U4 LDOs with switching power supplies. You are going to dissipate a lot of heat in the LDOs if you draw more than a few hundred mili-Amps and you are running with a 12 Volt wall-wart. Assuming 0.3 Amps on the 5 Volt supply –> 12-5 x 0.3 = 2.1 Watts. Thats a lot of power for those small parts and if you don’t have a large enough heatsink in the copper on the board they will probably get very hot. If you change to a switching power supply like the AOZ1280, you are good for at least an Amp and you will dissipate a lot less power without having to worry about heatsinking. That should give you more flexibility in the power draw for your daughter boards. The main downside with this approach is you will have increased voltage ripple on your power supplies, but with plenty of carefully placed bypass capacitors and series filtering (inductors, ferrites) you can minimize any problems that causes.

    3) As for selecting different power supply voltages, there are many ways to do this. If you are going to use the method of selecting different resistors with Atmega2560 I/O pins as you did in this design, I would recommend isolating the I/O pins from the regulator with N channel FETs. At least if you do that, you don’t leave the possibility of any current from the regulator getting in through the static diodes of the Atmega2560 and causing problems. Just drive an N FET gate with the I/O pin, put the source to ground, and put the drain to your selection resistor.

    Another way you could accomplish variable supply voltages, and one that would give you a lot more control over what voltage you get out, is to use a digital potentiometer. These can be controlled over SPI or I2C and will essentially allow you to set your supply voltage to anything you want. If you put a regular resistor divider for the feedback on the regulator, and then put the digital potentiometer with the wiper terminal on ground and the A or B terminal at the center of the divider you can set the feedback voltage by changing the digital pot value. You just have to be careful that in software you don’t go past the bounds of allowed supply voltages. But you could easily control that by writing a software loop to step the potentiometer resistance and then check the reading that you got with your pretest circuit, until you get to the desired voltage. If you’re careful about how you write your code you would essentially have a programmable power supply. And if you use a DC-DC converter your output could range anywhere from 0 to 16V (your max listed input voltage) at 1 Amp output current. That would likely be more than enough range for most of your test boards.

    4) I would recommend driving your pretest circuits (signal “44/PT_CTRL”) with a high side P FET switch or at least an op-amp configured as a buffer to isolate the Atmega2560 I/O pin from V1 and V2. You don’t want the microcontroller to sink current (albeit very little) on these lines. I would also use an op-amp buffer on “A14/PT_READ_V1” and “A15/PT_READ_V2” to isolate the microcontroller ADC from these signals. Isolation is never a bad idea except for the fact that it takes more parts. You probably aren’t going to make a million of these boards and I would guess that reliability is more important than a few extra dollars per board.

  • It appears that the RFD77402 used on the distance sensor breakout is obsolete (according to Digi-Key and Mouser anyway). Are you aware of this and if so do you have any suggestions for a drop in replacement in the future?

  • It appears that the RFD77402 used on the distance sensor breakout is obsolete (according to Digi-Key and Mouser anyway). Are you aware of this and if so do you have any suggestions for a drop in replacement in the future?

No public wish lists :(