Attiny85 Buck Converter for hearing Aid

[jalilurrehmaan9]

Thanks Every body for Suggestions.
I agreed my line "Need help to solve power issue for hearing Aid" caused confusion. All I am trying is to convert 3.7 volts of Lo-Po battery to hearing aid compatible 1.47 volt.
I calculated efficiency of different solutions and results are following
Assumptions: Some values are rounded to keep calculation simple.
Battery minimum voltage = 3v
battery max Voltage = 4v
I _load = 3mA
V_{load =} 1.7 v
**1. **** at 100 % efficiency using black magic **
Battery capacity = 3.7 v * 1000mAh = 3700mWh
load power = 3A*1.7v = 5.1mW
@ 100 % efficiency
time = 3700/5.1 = 725h = 30 days not bad

2. using LM317 linear voltage regulator
if V battery = 3v
I _in = 9mA
V _load 1.7V
I _load = 3mA
Efficiency = (31.7)/(39) = 18%
expected battery life = 30 Days * 0.18 = 5.14 days

if V battery = 4v
I in = 9.67 mA
V load = 1.85 v
I out = 3mA
Efficiency = (1.853)/(9.674) = 8.59 %
Expected battery life = 30days * 0.085 = 2.41days

3. using Attiny85 **
Current required to power Tiny = 2mA
At 4v
Power required for tiny = 8mW
let suppose Switching circuit and LC filter is 80 % efficient and 20 % is loss in heat
total power input = 8mW + 80% 5.1 mW = 12mW
efficiency = 8/12 = 66.6 %
Expected life = 19 Days ** Attiny85 based buck converter is higher efficient linear one.*

**4. RT8059 Buck IC ** Luckily I found this IC near me.
Datasheet claims 95% efficiency
Suppose effective efficiency is 90 % after loss in LC FIlter
Expected battery life = 80 * 0.9 = 27 days

5. Zener based voltage regulator
I think zener Would have same efficiency as LM317 or may be Worst.

This li-po battery won't hang in ear. It will be in pocket a thin wire will carry power to hearing and will also keep it captive. Of course I am planned to used Battery management system and A poly fuse to prevent Battery damage and Potential problem. also order some Zener diodes to clip any over shot in voltage.
Let me Know What you guys think and thanks for suggestions.

[wvmarle]

jalilurrehmaan9:
Thanks Every body for Suggestions.
I agreed my line "Need help to solve power issue for hearing Aid" caused confusion. All I am trying is to convert 3.7 volts of Lo-Po battery to hearing aid compatible 1.47 volt.

That's one option; another option is to simply use an AA alkaline or NiMH battery and be done with it there and then. Cheap, quick and simple. But of course, where's the fun in that (other than helping your grandpa to hear now instead of when you're done hobbying).

2. using LM317 linear voltage regulator
if V battery = 3v
I _in = 9mA
V _load 1.7V
I _load = 3mA

Input current = output current + ground current; the latter is usually negligible. So for 3 mA output you need 3 mA input. Efficiency is 1.48/3.7 * 100% (using the nominal voltage of the LiPo) = 40%.

3 mA power draw gives 1230 hrs or 51 days of battery life.

3. using Attiny85

Which will have to run at 8 MHz and battery power so expect about 5 mA. That is already almost double the linear regulator takes so I'm stopping my calculations here. It's a worse solution.

4. RT8059 Buck IC Luckily I found this IC near me.
Datasheet claims 95% efficiency
Suppose effective efficiency is 90 % after loss in LC FIlter
Expected battery life = 80 * 0.9 = 27 days

To get 90% efficiency I must assume you are getting a complete ready-made module. That would be a good solution, if you have such a module, but then I assume you don't as otherwise you wouldn't have to ask to begin with. Hand soldering dead bug style or on protoboard is just about impossible with this chip (it's too small a package), a PCB is a must. Make sure you closely follow the suggested layout in the datasheet, and get the recommended parts (the exact type of inductor and capacitor matters, big time).

According to the datasheet you can not expect more than about 80% efficiency for your low power draw and output voltage.

In this case: 1.48/3.7 * 3 / 0.8 = about 1.5 mA draw from the LiPo, plus quiescent current (almost negligible at 78 µA, pretty good). That'd give a battery life of just over 100 days.

5. Zener based voltage regulator
I think zener Would have same efficiency as LM317 or may be Worst.

Definitely worse than the LM317 as for proper regulation you must feed more than the peak draw of the hearing aid to the zener, and that is continuous. A linear regulator only draws the current needed, and is able to supply a far more stable voltage output.

Conclusion: use an AA battery. Or if you must use that LiPo, wire up a linear regulator.

[jalilurrehmaan9]

"

Input current = output current + ground current; the latter is usually negligible. So for 3 mA output you need 3 mA input. Efficiency is 1.48/3.7 * 100% (using the nominal voltage of the LiPo) = 40%.

3 mA power draw gives 1230 hrs or 51 days of battery life.

Sorry I didn't got calculations.
how 3mA current draw from 1000mAh battery give 1230 hr or 51 days?
I am curious to Know where I made mistake in calculations

That's one option; another option is to simply use an AA alkaline or NiMH battery and be done with it there and then. Cheap, quick and simple. But of course, where's the fun in that (other than helping your grandpa to hear now instead of when you're done hobbying).

I think their is a reason for not using AA battery which is AA are bulky and don't sit flat like Li-po battery and of course some adventure in Electronics Online classes are boring need some thing to spend time.

To get 90% efficiency I must assume you are getting a complete ready-made module. That would be a good solution, if you have such a module, but then I assume you don't as otherwise you wouldn't have to ask to begin with. Hand soldering dead bug style or on protoboard is just about impossible with this chip (it's too small a package), a PCB is a must. Make sure you closely follow the suggested layout in the datasheet, and get the recommended parts (the exact type of inductor and capacitor matters, big time).

Yes! I successfully Etched PCB for this IC and and practiced on scrap boards. I also used AC dimmer to reduce temperature of soldering iron to prevent pad lifting. I ordered parts Recommended in datasheet and also added poly fuse and Zener diodes for protection.

and Thanks

[wvmarle]

Oh, I was confused by your calculations, using 3,700 mAh instead of 1,000 mAh. That's why the calculations are off.

If size/portability is an issue... try beating a AAA with your 1,000 mAh LiPo and additional hardware in both weight and size. Battery life will be similar - an alkaline AAA delivers some 700 mAh, but no conversion losses. You probably can get away with a single LR44 alkaline cell, even smaller and lighter! But of course much shorter battery life.

[Paul_B]

Or just pull a 9 V Alkaline apart and use the "AAAA" cells it contains; as they have welded connector strips you can easily solder to them; no holder required! Cheapest, lightest of all.

A LR44 is almost as small as the Zn-Air anyway, and the economics may not be much better.

[wvmarle]

Paul__B:
Or just pull a 9 V Alkaline apart and use the "AAAA" cells it contains;

Not all of them are build up like that; all the PP9 batteries I've dismantled so far had a vertical stack of 6 rectangular cells... It seems to be brand specific.

[Paul_B]

Was it an alkaline though?

The use of rectangular cells was the original "minimax" battery design from the early 20th century.

[wvmarle]

I forgot. The next time I have my hands on a dead alkaline PP9 (they're too expensive to sacrifice new) I'll definitely have to take a look inside.

My latest battery dismantling, not too long ago really, was to obtain carbon electrodes. That's when I learned that alkaline batteries don't have those...

[Paul_B]

wvmarle:
My latest battery dismantling, not too long ago really, was to obtain carbon electrodes. That's when I learned that alkaline batteries don't have those...

Ah! I didn't know that!

Further research pending ...

[Paul_B]

Paul__B:
Further research pending ...

Fascinating!

This "breakdown" is interesting.

The alkaline batteries not only do not have a carbon rod, but the central pin is actually the negative connection to the Zinc paste!

[jalilurrehmaan9]

Thanks every one. I successfully etched PCBs and soldered all components. After soldering I checked efficiency of buck converter it was about 58%. Problem was high resistance of inductor I replaced it that's why there is a bigger Inductor on PCB. Now efficiency is 80%. First hearing aid produced some noise, a capacitor on input of hearing aid fixed these problems. Now everything is working fine. I attached picture of PCBs. feel free to Comment on it.

[wilykat]

Just asking since I am deaf and wear hearing aids. My battery lasts about a month. Are you buying air activated version (has tab to peel off), those are supposed to last longer than a few days with hearing aids. Batteries without tab are already active from the moment they are assembled while those with tab, they are inactive until the tab is peeled and air gets into the hole to start chemical reaction producing electricity.`