# Stepping up 3V to 5V

Can you help me with a way to step up a voltage of about 3.29 to 4.5/5V. I need a simple and efficient circuit to implement this. I will appreciate any useful links.

Thanks!

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How much current do you need? –  stevenvh Aug 24 '11 at 14:10
I need a current of 3A –  Paul A. Aug 25 '11 at 8:28

The first solutions given are low power really, because I didn't know yet what the current needs were. A higher current solution answering Paul's actual needs is given at the end of this answer.

You don't say how much current you need from the higher voltage, but I'll presume it's for a power supply, say at least 100mA.
For this an SMPS (Switch-Mode Power Supply, aka "switcher") is usually used. SMPS basically can be divided in two groups: buck (or step-down) and boost (or step-up) converters. To go from a lower to a higher voltage you need a boost converter.

The example shows a boost converter to get 3.3V from a pair of AA cells, but this can be easily adapted to a 5V output; all you have to do is change a resistor value.
So we see that a switcher needs a few more components than a classic linear regulator like the LM78xx, but it's not that bad.

If you only need little current you may consider a charge pump voltage doubler.

You can have then a LDO (Low Drop Out) regulator to go from the 6.6V to 5V. Keep in mind that the output voltage of a charge pump drops rapidly with increasing current drawn.

edit
If you need 3A as noted in your comment the above proposed solutions won't do. You can use a National Simple Switcher like the LM3478 or one of these Linear devices. National's Webench tool created a design with a BOM cost of USD 4.36, Linear tends to be a bit more expensive.

More:
Boost regulator selection guide at Linear Technology

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I need a current of 3A –  Paul A. Aug 25 '11 at 8:28
Actually I need it to drive a MIC29302WU LDO. –  Paul A. Aug 25 '11 at 9:22
@Paul - Why don't you use the boost converter to get the desired output voltage directly? The LDO is not really necessary and it would considerably increase efficiency. The MIC has a 600mV max dropout, which means that you'd lose 1.8W in it. If the input voltage can be either lower than or higher than the output voltage you need a buck/boost regulator. –  stevenvh Aug 25 '11 at 9:44
Yea. I thought as much! Because the MIC will still have to bring to about 3.9V which is the real voltage for my intended application. Do you still think the LM3478 will do? These are the parameters I need; INSTANTANEOUS Current= MAX 2A; Working Current=210mA; Working Volatge= 3.5-4.8V(Ideally 3.9V). Do let me have your suggestion. Thanks –  Paul A. Aug 25 '11 at 10:51
@Paul - The LM3478 will do if Vout is always greater than Vin. Check out National's Webench designer. You supply input and output volatges and output current and Webench creates a design for you, including BOM. –  stevenvh Aug 25 '11 at 11:05

You can use a boost converter.

This is a circuit composed of oscillator, switching transistor, diode and an inductor/capacitor. Usually there is a feedback element to regulate the output voltage. The inductor performs the voltage boost.

If you have a look on RS/Farnell/Mouser/Digikey or similar and type in "boost regulator", or "boost converter", then filter by 5V output voltage, you should get lots of options.

Here is one at random from Farnell:

Boost converter example part

Another option is a charge pump IC (uses switched capacitors to double/triple/etc voltage), but these do not (generally) regulate the output voltage, although you can add a linear regulator after the charge pump. Or a transformer with necessary ratio of around 1:1.5, you switch the DC 3.3V into the primary to create the 5V, then rectify and filter to create DC.

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As people have noted,the name for the function that you describe is a "boost converter". There are a very large number of these available, with quite a few suited to your requirement.

If efficiency is a major consideration you would get best results using a synchronous rectifier in place of the output diode which is otherwise used. A synchronous rectifier uses an internal MOSFET to turn on when required to feed power to the output. The MOSFET has a lower voltage drop than can be achieved with a diode - even a Schottky diode which is what is otherwise usually used. Without a synchronous rectifier you can expect efficiencies in the 70% - 80% range with 3V3 input and 5V output - maybe more in some cases at some currents.

It is not possible to recommend an appropriate part with certainty until you advise output current requirement and ideally , what your application is, but as a guide, the cheapest synchonous output boost converter IC in stok and available from Digikey in 1's is the L6920DC 'Sychronous Rectifier stepup converter' made by ST.

This has a minimum of 550 mA input current. It will produce 300+ mA at 5V and probably quite a bit more. As can be seen, it would be hard to get a much simpler circuit:

At 3.3V in this IC will provide around 90% efficiency for output current from 1 mA to hundreds of mA.

As shown this makes a fixed output of 3.3V (not useful here) or 5V. Adding two resistors allows you to 'program" the output voltage to any voltage in the 1.8V to 5.5V range (as long as Vout > Vin. (Unlike many IC's of this sort, the one version allows 3.3V or 5V fixed output or variable voltage output). .

As above, whether this meets your needs depends on your power requirements. This IC will start on less than 1 Volt input. Not needed in your case but nice to know.

This part uses a TSSOP package which is not beginner friendly, but there are similar parts in larger packages.

A good place to look for ICs is Digikey's catalog In this case search for eg

• boost converter or

• boost converter 3v 5v

and then "drill down" and refune the parameters that matter to you. Synchronous rectification is not a selectable parameter but you can search for "synch" when you are viewing the tables.

For a head start, here's what you get from

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I am looking at a 5v,3A output. I am having a 3.29V from a prototype board, and I need to drive a MIC29302 regulator, which requires 4-5V, 3A. –  Paul A. Aug 25 '11 at 9:52