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I'm thinking about connecting a micro SD card to my ATmega162 perfboard, but for that I need 3.3 V source which can provide up to 100 mA.

Most obvious option would be to run the whole board at 3.3 V because all components seem to be working at 3.3 V, but I'd like to keep the 5 V input voltage.

Next option and the most obvious one would be to use voltage divider. The downsides of that would be need for high precision resistors, bad stability, space taken and power consumption.

Another option would be to use a 3.3 V regulator. After checking which regulators I can easily obtain, choice comes down to LF33CV or LP2950CZ-3.0.

The LF33CV fits from the electrical side with its dropout voltage of 0.45 V and very low power consumption. It can also provide up to 500 mA, which should be enough for most micro SD cards. I'd need two capacitors, one 0.1 µF on input and one 10 µF at output. On the other hand, the regulator is TO-220 and I'd like to save as much space as possible on the board.

The LP2950CZ-3.0 is under just acceptable category from the electrical point of view. It has dropout voltage of 0.38 V and is guaranteed to provide up to 100 mA, which is exactly the amount I expect to need. On the other hand it's much smaller and comes in TO-92 package. It also only needs one 3.3 µF capacitor at its output in order to work properly.

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  1. Why use 5V if you don't need to?

  2. 100 mA * (5-3.3) is 170 mW. No big deal for TO-92.

  3. A cheapskate hobbyist approach is three silicon diodes in series. This will get you in the neighborhood of 3V. Might need to add a load resistor to set the max voltage with no card.

Have you thought about how you will level-shift the signal lines? SPI lines are all unidirectional, which should simplify things. 3.3V is a logic "high" for a 5V input. The outputs should be level-shifted somehow. A two-resistor divider would work ok. Use a 1.5k and 3.0 k resistor to get pretty close to 1000 ohms output impedance. You'll need this on clock, MOSI, chip select at least.

Related: How do I get an Arduino (5 V) and MPR121 (3.3 V) to talk?

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Go with all 3v3 if you can.

If not, interfacing between 5v and 3v3 can be done with 74LCX series glue chips.

You can get 3.3v buffer chips with 5v tolerant input such as the 74LCX244 (the X in the code means 5v tolerant). You can hook these straight to 5v outputs and they give 3v3 outputs. (you'll need a 3v3 regulator to power the chip, a teeny TO92 5v->3v3 one is fine, and about 20c).

In the other direction (3v3 to 5v) you can just use pullup resistors to go from 3v output to 5v input.

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A voltage divider is NOT a solution in this application.

Really, a voltage divider is not a solution anywhere you have any variance in the current drawn, or really in any power section of anything.

Also, think about the resistor sizes you would need to make a divider without significant droop when 100ma is drawn from it.


Now, a series resistor will work in cases where the drawn current is extremely fixed, but think about what would happen if you were to change the current load - the voltage would increase when the current decreases, and vice-versa.

You really HAVE to use a voltage regulator here.

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  • \$\begingroup\$ The stiffness of a two-resistor voltage divider is the resistance of the two resistors in parallel. To get 300 mV drop when 100 mA is drawn, this parallel resistance must be 3 ohms. If R1 is the top resistor and R2 the bottom, then R1=Rp / k, where k=3.3/5.0. And R2 = k*r1/(1-k). So the divider would be 4.55 and 8.82 ohms. Quiescent current 374 mA, burning off 1.87 watts. A voltage regulator seems like a better idea. \$\endgroup\$ – markrages Feb 27 '11 at 4:44
  • \$\begingroup\$ @markrages - Yeah, I just didn't remember the formula off the top of my head. \$\endgroup\$ – Connor Wolf Feb 27 '11 at 4:58
  • \$\begingroup\$ to be honest, I didn't either and had to write it down. \$\endgroup\$ – markrages Feb 27 '11 at 5:00
  • \$\begingroup\$ @markrages - well, that makes me feel better. \$\endgroup\$ – Connor Wolf Feb 27 '11 at 7:41

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