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.
Best Answer
Why use 5V if you don't need to?
100 mA * (5-3.3) is 170 mW. No big deal for TO-92.
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?