Power supply options

Community PCB Design
1

The schematic below shows the part of the circuit used for power supply. (circuit with PIC32MX795F512L)

There is 3 possibilities:

  • 3.3V applied directly to TTL

  • 5V applied directly to TTL

  • 5V from the USB

To choose on of those options user must connect/disconnet the 4 jumpers.

To use the 1st power option: JP1, JP3, JP4 disconnected while JP2 connected.

To use the 2nd power option: JP2, JP4 disconnected while JP1, JP3 connected.

To use the 3rd power option: JP2, JP3 disconnected while JP1, JP4 connected.

Is that good or should I add some components to the schematic?

2

That would work, but gives too many chances to get things wrong.

This version will reduce the chances (but still leaves the option of feeding 5V on the 3.3V rail - maybe add a zener to take care of this?) It’s based on using standard 0.1" headers and 2-pin shunts or the equivalent.

The 3-pin block will let you select the TTL connector or USB connector as the source of power (and prevent you from connecting both at once). The 4-pin block with jumpers placed vertically would tie in the regulator for 5V input, and with jumpers horizontally, would bypass the regulator for 3.3V input.

/mike

3

Well in your version there is 4 possibilities of connecting the jumpers and 6 possibilities to apply power:

1- 3.3V via TTLV and horizontal 4 pins block

2- 5V via TTLV and horizontal 4 pins block (not good)

3- 3.3V via TTLV and vertical 4 pins block (not good)

4- 5V via TTLV and vertical 4 pins block

5- 5V via VUSB and vertical 4 pins block

6- 5V via VUSB and horizontal 4 pins block (not good)

To solve case number 3 I figured out a circuit:

In case we apply 5V via TTL or USB:

C = 5V

Vz = 3.9V

B = 5V - 3.9V = 1.1V > 0.7V -------------> E = 5V

In case we apply 3.3V via TTL:

C = 3.3V

Vz = 3.9V ---------> B = 0V ----------> E = 0V

Is this a good circuit? Now how I can solve the 2nd and the 6th cases?

4

Hello,

I want to use a power protection circuit on my board letting only 3.3V pass through (the other possibility is 5V that I want to block). Is the schematic below correct? Is it dangerous (especially for the Zener)? Is there any other suggestions/modifications or other advised schematics that can do the same job?

Thanks for your help!

5

First, let’s keep this topic on one thread; spreading it out is frowned upon, and is hard to follow.

I’m not sure crowbarring the input rail is what you want. If you do use a crowbar, add a fuse so that it will blow before something else.

What about using a buck/boost regulator instead of all of this. The LM3668 will work on 2.8 to 5V in, and give you 3.3v@1A. Precede it with an diode OR (or better yet, a FET OR like the TPS2115A) to automatically select an input.

/mike

6

Okey, let’s keep the conversation here, sorry.

In fact, I will use the circuit u proposed with the 3-pins jumper and the 4-pins jumper. To prevent the option of feeding 5V on the 3.3V rail (5V via TTLV and horizontal 4 pins block OR 5V via VUSB and horizontal 4 pins block) I will use the circuit I posted above on the 3.3V rail.

For your second proposition do you have a shematic or something illustrating that?