5V to 3.3V

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5V to 3.3V

Post by Spymat » Tue Feb 05, 2002 3:22 pm

Hi,<p>I'm using a board that requires 5 Volts and 3.3 Volts. I can only supply the board with 5 at this moment. Are there any IC's that I can input 5 volts and get 3.3 volts for the chips that need it? The chip that requires the 3.3 is a Xilinx 4000 Series FPGA<p>Kind Regards,

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Re: 5V to 3.3V

Post by jollyrgr » Tue Feb 05, 2002 5:18 pm

You can build a simple regulated voltage supply out of a transistor, a couple of resistors, and some diodes. I'll try to describe a suitable circuit for you. (Any corrections, hints etc. would be welcome.)<p>1) Pick a NPN transistor that can handle TWICE the current you will ever draw from the 3.3V
2) Connect the Collector of the transistor to the +5 volt supply.
3) Connect one end of a 100 ohm resistor to + 5V. Connect the other end of the resistor to the CATHODE (banded end) of a 3.3 volt Zener diode.
4) Connect the ANODE of the Zener diode to ground. (This forms a reverse bias diode circuit. You will be using the 3.3 volt break down voltage to form your regulator).
(Alternately, you could use two RED LEDs in series in a forward bias configuration instead of the Zener diode. Measure the voltage across the diode for a "close" voltage of what the regulated voltage will be.)
5) Connect the base of the transistor at the junction of the zener diode and its bias resistor.
6) Connect a 1K Ohm resistor between the transistor EMITTER and ground. Put a cap (100uF???) across the resistor as well.
7) Take the regulated 3.3 voltage from the EMITTER to ground.<p>While this circuit is by no means a well designed voltage regulation system (you have not provided enough information), it will work to a fair degree. Included is a URL for the circuit I just described (see the descrete regulator section). While the values on this schematic are for a different purpose, the values I give are my "best guess" for the circuit you need.<p>http://www.ee.nmt.edu/~cww/courses/ee32 ... _lab1.html
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Re: 5V to 3.3V

Post by russlk » Tue Feb 05, 2002 7:19 pm

You can do it with one TO-92 IC: LT1121CZ-3.3 available from Digi-Key for $2.38.<p>Note: Digikey.com has a great search engine when you are looking for something like this.

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Re: 5V to 3.3V

Post by Externet » Tue Feb 05, 2002 9:29 pm

Hi Matthew.<p>Almost any defunct computer motherboard will be a glad donor for such low dropout voltage regulator.
They are usually TO-220 with a small heatsink, and can supply up to 3 Amperes.<p>Computer power supplies yield 5V, but many chips on the mainboards run on 3.3V.<p>Miguel
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Re: 5V to 3.3V

Post by viveguy » Wed Feb 06, 2002 7:36 am

Sign up for the Maxim news letter and order a free sample. They have scads of different types.

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