[Divedeep]

Hi guys,

Was wondering if you might be able to help.

I'm working on a project where i have a Li ion power source. This has to power various sensors and peripherals at 3.3v.

What i would like to happen is while the battery is fully charged then the voltage is stepped down to 3.3v. When the battery begins to fall below 3.3v then it is boosted to maintain 3.3v.

I was hoping to use a buck boost converter but cant seem to find anything to fit the bill. I have not used one of these before so not exactly sure what to look for.

I have a current requirement of around 200ma so i would like to have a converter capable of 300-350ma so as not to stress the converter.

Can anyone help me out in finding something suitable?

 

[(*steve*)]

Get onto eBay and search for "buck boost".

You'll find hundreds For 200mA they shouldn't be stressed too much, although if you want to ensure the best performance and quality you should perhaps choose something other than a cheap Chinese board.

The problem you might have is finding one that will work with an input voltage below 3.3V (e.g. this has a minimum input voltage of 3.5V )

Some others will start with lower voltages but the topology they use is inefficient.

It is possible that some of these units will operate at lower voltages than they can start at.

A brief search didn't reveal any single chip solutions for a 3.3v power supply from a single Lithium cell (I'm somewhat surprised)

 

[Divedeep]

Hi Steve,

I was also surprised that there doesn't seem to be a solution for a single cell Li ion battery. You would have thought due to Li ion being quite prominent that there would be.

The ebay route is not really an option for me, this regulator needs to sit in a completed circuit.

I am not really familiar with batteries and there inner working as i usually use fix power supplies for power. Is there a way of calculating how much power is left in a Li ion battery at a particular voltage??

If i for instance used a simple LDO regulator that has a 100mv overhead, this would mean that my circuit would stop working at about 3.4v. At 3.4v how much power would a Li ion cell have left in it??

The cell i plan to use is a 1800mah 3.7v Li ion.

Im thinking if i could get 8 hours use at 200ma draw this would be satisfactory for me. Also i expect that the higher cutoff point would keep the cell healthy and it would last much longer.

 

[shumifan50]

I seem to remember a presentation about Li-ion batteries where it was recommended that the baterry not be discharged below 50% to improve life. The level of discharge and temperature during charging affected baterry life most.
Following that logic your battery should not be discharged for more than 4.5 hours at 200ma.
Unfortunately I also am not experienced with batteries.

 

[Divedeep]

In practice the battery will not be discharged at 200ma for 8 hours. It will have periods where the load is 200ma and at others the load will drop to less than 50ma. i am working on a worst case basis and anything better in the end is a bonus to me.

Also the device itself will probably never be on for more than an 2 hours at any given time.

 

[BobK]

At low discharge rates, most of the battery capacity has been used at 3.5V. So if you can find a 3.3V regulator with a dropout voltage of 0.2V you would be fine.

Bob

 

[Divedeep]

I was looking at the LTC3440, it seems to be used quite a bit, its a buck boost converter so suits my application quite nicely.

Would there be any benefit to this over a standard LDO regulator??

Also does anyone know what i would have to do with the sync pin? In the schematic it shows it floating. On many schematics in Google searches it doesn't even show in in the design. I assume it is tied either high or low ???

 

[Arouse1973]

I think you need a SEPIC converter.
Thanks
Adam

 

[(*steve*)]

I think you need a SEPIC converter.

Other than low efficiency and black magic, they're really cool

 

[OLIVE2222]

Hi,

i am with Bob, because of the battery abrupt end of life curve you can go for an LDO with an output value close the lower possible working voltage. Can even be lower than 3V3 if the board is self contained (i.e. no need for extenal 3v3 I/O) maybe 2V7 or 3V.

Olivier