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Author Topic: [SOLVED] Which Voltage Regulator use ?  (Read 6610 times)
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Hello everybody,

A friend and I are working on building a smartwatch on a budget, so it’s a (very) low power project, but we are currently having a problem with the power supplying.
We already chose this board, because of its size and the BLE chip: RFduino BLE SMT
For now on, we are planning to use this battery: 500 mAh LiPo battery (approx. 4V at full charge) with this voltage regulator: Micrel MIC5209-3.6Y (the board supports 3.6V max.).
 
But the problem is that the Voltage Regulator will apparently drain by itself too much energy from the battery (by heating), reducing drastically the autonomy.
 
So we would like to know if you guys can help us, if you can tell us exactly what (type of) regulator would be the best or if we need to find another battery.
 
Thanks smiley
« Last Edit: April 26, 2014, 07:58:29 am by Black3v3r » Logged

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Try one of these
http://www.pololu.com/category/131/step-down-voltage-regulators
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The OP is looking for a regulator with ultra low quiescent current (20uA or less I guess).

I'd query the need for a regulator at all - use LiFePO4 cell and you have ~3.3V supply
and no chance of battery bursting into flames...  Finding a LiFePO4 in the right form factor
may be tricky though.
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These are kind of expensive but using a buck-boost ic will allow you to use more of the battery life:

http://www.mouser.fr/ProductDetail/Texas-Instruments/TPS63020DSJT/?qs=sGAEpiMZZMvAX9OfPh%252b2NZMDmsz7q2UaVBssz51T%2fhY%3d

The problem I find with standard regulators (linear or switching) is you usually need to allow for at least 0.5 volts above the regulated voltage.

Tim
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Hi,
Thank you for these very fast answers !

CrossRoads : Those are switching regulators, right ? I've heard that they are more power efficient (80 to 95% according to your link). Correct me if I'm wrong, but when the battery voltage go below 3.3V, the regulator don't let the current pass through ? This is kind of an issue, because the board can run down to 1.9V. (Even if the battery protection circuit cut the power before 1.9V, we can go further than 3.3V).

MarkT : we have also looked for this kind of battery but, as you pointed out, it is impossible to find a battery small enough for the project. Maybe we have missed one. Do you have any reference that could fit our needs ?

TatankaWillly : That one seems very interesting ! I didn't know a single chip could do both ! The quiescent current of this chip is around 25uA according to the datasheet. It seems a bit high to me, knowing that the e-ink screen we will use only drain 4uA. I know it is still not much (uA...) but do you think we can find a chip with a lower quiescent current ?
« Last Edit: April 24, 2014, 12:39:38 pm by Black3v3r » Logged

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Take a look at this one
http://www.pololu.com/product/2120
Set the output to 2V.
Adjust the regulator to shutoff if the battery voltage gets too low.
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Thank you very much for the link. The shutdown pin would be useful ! According to the description, the efficiency with Vin=3V, Vout<3.3V and Iout=50mA  would be around 40%. Don't you think it will affect the battery life ?
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I just found this regulator : http://www.mouser.fr/ProductDetail/Texas-Instruments/LP2985IM5-36-NOPB/?qs=sGAEpiMZZMsGz1a6aV8DcBojudwDyMGaQ7IKehkX01U%3d
Do you think it would be a good choice ?
I'm expecting it to work as follow :
When Vin>3.603V, Vout is 3.6V
When Vin<3.603V, Vout is Vin-0.003V
Is it a good guess ?
Thank you.
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But the problem is that the Voltage Regulator will apparently drain by itself too much energy from the battery (by heating), reducing drastically the autonomy.
 
So we would like to know if you guys can help us, if you can tell us exactly what (type of) regulator would be the best or if we need to find another battery.


You're getting a lot of options.  What are some of of your other requirements?  Cost?  Size/package of the regulator?  There are a ton of chips that do "step up" and "step down", which is what you want it seems.

Can you use surface mount parts or do you have to stick with thru-hole?  If you can use surface mount options, there is a lot of "dc dc converters".  You can borrow the design from the "Jeenode battery module", which uses LTC3525.  It's a surface mount, draws 7-30uA of wasted current, accepts input voltages from 1V to 5.5V which is within your battery specs.  It also has an output enable pin.  Best of all, unlike many other dc-dc converters, the external components you need to add (input cap, output cap, inductor) are not as expensive as other dc-dc converters.

If you can't use surface mount components, try something like MAX756EPA.  But this needs fancier capacitors and a zener diode.
« Last Edit: April 24, 2014, 03:23:56 pm by arusr » Logged

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The LP2985 does look like a good option. Be aware that SOT chips are pretty small.

I'm expecting it to work as follow :
When Vin>3.603V, Vout is 3.6V
When Vin<3.603V, Vout is Vin-0.003V
Is it a good guess ?

No. Page 4 of the datasheet lists the "dropout voltage" for various output currents. As an example, at 50ma current the dropout is 120mv typical; that means that once your battery drops to less than (3.6V + 120mv =) 3.72V the regulator just turns off. Due to that I would recommend a 3.3V regulator for a LiPo battery.

The efficiency of a linear regulator is based on the difference between the input and output voltage. So if your battery is at 4.2V and you're outputting 3.3V then you're (3.3 / 4.2 * 100 =) 78% efficient. I would not complain about that. LiPos spend most of their life between 3.8V and 3.6V, so at 3.3V output the efficiency is higher.

A typical LiPo cell is about 95% discharged when it reaches 3.4V (in a situation like yours where the rate of current draw is low). You don't really want to go any lower or you'll be killing the battery. Again that's why a 3.3V regulator is perfect for LiPos.
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Great ! Thank you very much ! We'll try LP2985 then.
Thank you again !
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