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Author Topic: Opposite of a voltage divider?  (Read 1363 times)
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Hi all,

I'm building a circuit that uses PWM to drive a mosfet.  I hava an Arduino set to pump out a 200Hz square wave, and all is well.  This works fine with the 5V pins on the Arduino.

However, another project calls for the exact same mosfet circuit and frequency, however I'll be using a ChipKit UNO, which is a 3.3V board.  The PWM from the 3.3V pins is not sufficient to drive the mosfet.

So, I'm wondering, is there any sort of simple circuit that will act as a voltage divider, but in the opposite sense to drive 3.3V up to 5V?  I know about boost converters but I'm thinking that's a little too much complexity for my application.
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It's easiest to just use a MOSFET with a lower threshold voltage, for example this guy:

http://search.digikey.com/us/en/products/IRLML2402TRPBF/IRLML2402PBFCT-ND/812508

It has a guaranteed maximum on-resistance of 0.35 ohms at 2.7V.

Or you can get a MOSFET gate driver like a TC4427 to function like a "reverse voltage divider", i.e., taking in a low voltage and putting out a higher one. But you will need to power it with the higher voltage that you want.

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Use the 3V PWM pin to switch a bipolar transistor to switch the gate voltage for the MOSFET.
You didn't state what your MOSFET +V.
I've posted this circuit before --
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If you've got 5V available so you don't have to generate the 5V supply, it's fairly easy to boost a 0-3.3V logic circuit up to 0-5V.  Interfacing 3.3V logic to 5V logic  (and vice-versa) is a common problem.

A quick-search turned-up 74LVC4245.
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So, I'm wondering, is there any sort of simple circuit that will act as a voltage divider, but in the opposite sense to drive 3.3V up to 5V?

There is such a circuit called a "voltage multiplier" - you typically see them used in cascades for high-voltage work (ie - building things like tasers and stun-guns)...
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There are specific devices designed for just this task.

They are called "Logic Level Translators".

They convert 3.3V to 5V and 5V to 3.3V for logic signalling between devices of different voltages.  There are also 1.8V versions as well, and universal ones where you just supply both voltages as inputs and it converts between them.
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See also "MOSFET drivers"...
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