[SOLVED] 2xAA (~3.4V) to 5V step up circuit

[nhoratiu]

Hello everyone,
Ive been trying to build a dc-to-dc step up circuit that uses two AA batteries (~3.4V) to power a 5V RFID reader unit.
I'm using a MCP1700-5002E but it doesn't seem to be working. Whatever I put in under 5V will be matched on the other side. For example, placing two AA batteries (~3.4V) at Vin will be ~3.4V at Vout. Placing 4xAA batteries (~6.2V) makes it 5V at the output.
What am I doing wrong? Aren't these MCP1700s supposed to step up the voltage to 5V?

Thanks

 

[keith1200rs]

That regulator is a low dropout linear regulator, not a switching regulator. All it can do is reduce the voltage and will dissipate the difference in heat.

If you want to increase the voltage you need a switching regulator.

Keith

 

[crutschow]

How much current at 5V do you need?

 

[Jigar 4 Electronics]

you can use simple voltage amplifier circuit to do so, and can use a potentiometer to control its gain which will give you output as per your need.

 

[keith1200rs]

you can use simple voltage amplifier circuit to do so, and can use a potentiometer to control its gain which will give you output as per your need.

That doesn't make sense. To step up the battery power you need a switching regulator. An amplifier as you describe is no use at all - it would need 5V power at least which is what the 2xAA batteries DOESN'T have.

Keith

 

[Jigar 4 Electronics]

But can't we use current amplifier after Voltage amplifier in the circuit ?
Well may be I am wrong because I don't have that much experience but just argued to know the answer of my query, hope you don't mind.

 

[keith1200rs]

But can't we use current amplifier after Voltage amplifier in the circuit ?
Well may be I am wrong because I don't have that much experience but just argued to know the answer of my query, hope you don't mind.

You would have to draw what you have in mind because your suggestion makes no sense to me.

To generate a high voltage power supply from a lower voltage you need either an inductive circuit i.e. a switching regulator or a capacitive charge pump circuit. An "amplifier" can only increase an input voltage up to the level of its power supply levels.

Keith

 

[Fragrance]

the best and great chip is max757 or TL499 although uploaded circuit for 9 volt but you can set out put divider for 5 volt

regards
Fragrance

 

[nhoratiu]

The RFID reader works at around 200mA. I should probably use several switching supplies in parallel (about 3-4) to get the required current.

Thanks!

 

[keith1200rs]

The RFID reader works at around 200mA. I should probably use several switching supplies in parallel (about 3-4) to get the required current.

Thanks!

You cannot do that - they will not share the current equally. You need to choose a suitable switching regulator. There are plenty of choices -200mA isn't a lot of current.

Keith

 

[phanikodali]

Hello,

follow this link for some charge pump regulators from texas
**broken link removed**

for boost regulators:
**broken link removed**

 

[keith1200rs]

I would suggest the switching regulators. While you can use a capacitive charge pump they are really integer voltage multipliers so you can double or triple your voltage for example. While there are some which do intermediate voltages (e.g. the TPS60130) they don't work at high efficiency with non-integer multiples and the input voltage range is more limited. The lower efficiency would result in more heat dissipation.

Keith

 

[FvM]

Charge pump circuits with fractional voltage ratio (e.g. 2:3) can work "lossless" as well. But for a battery input, you would want to keep constant output voltage over the full life cycle voltage course, about 3.2 down to 2.4 V, which demands for an inductive boost converter.

 

[nhoratiu]

Allright, I think I have a good idea of what I need now. I will look for a switching regulator and probably will be using either max757 or TL499. Thanks a lot!

 

[phanikodali]

But rated continuous output current of TL499 is 100mA, what you require is 200mA, so, it doesn't seems to be right choice, it would be better to choose a regulator rated at 250-300mA at least.
Coming to max757, it's specifications looks like matching your application, but its cost is little high, efficiency is less(efficiency is a critical parameter in battery powered devices) when compared to TPS61240-Q1, TPS61028. You can also try TPS60130 charge pump regulator, but its efficiency is too low and minimum input voltage is 2.7V(of course it may not be a big problem).