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Choosing right inductor...

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3dgeo:
Hello,

I'm making step down converter (based on "Great scotts" BMS, schematic below) for LiPo battery to replace 2 AAA batteries and my knowledge about inductors is a bit weak, so I need help.
MT3420 data sheet recommends 1-4.7uH with low DCR for higher efficiency, lowest DCR I managed to find is 4.2 mOhms@2.2uH(13x13x7 mm), but also I found 24mOhms@4.7uH(8x8x4mm). How much of an impact inductor inductance and DCR has in this particular case, load will be wireless mouse (will make separate thread on it in a few min...).

Thanks in advance.
(don't mind resistors values)

mvs:

--- Quote from: 3dgeo on January 14, 2019, 09:46:30 am ---MT3420 data sheet recommends 1-4.7uH with low DCR for higher efficiency, lowest DCR I managed to find is 4.2 mOhms@2.2uH(13x13x7 mm), but also I found 24mOhms@4.7uH(8x8x4mm). How much of an impact inductor inductance and DCR has in this particular case, load will be wireless mouse (will make separate thread on it in a few min...).

--- End quote ---
Power loss due DCR is P=I²R. If your mouse consumes 50mA, then it is 2.6µW vs 15µW. 

3dgeo:

--- Quote from: mvs on January 14, 2019, 10:40:30 am ---
--- Quote from: 3dgeo on January 14, 2019, 09:46:30 am ---MT3420 data sheet recommends 1-4.7uH with low DCR for higher efficiency, lowest DCR I managed to find is 4.2 mOhms@2.2uH(13x13x7 mm), but also I found 24mOhms@4.7uH(8x8x4mm). How much of an impact inductor inductance and DCR has in this particular case, load will be wireless mouse (will make separate thread on it in a few min...).

--- End quote ---
Power loss due DCR is P=I²R. If your mouse consumes 50mA, then it is 2.6µW vs 15µW.

--- End quote ---

Thank you.

Does inductance have a huge impact on this small load? Cos that inductor is 13x13x8mm, would like something smaller if it's possible :)

mvs:

--- Quote from: 3dgeo on January 14, 2019, 11:11:51 am ---Does inductance have a huge impact on this small load? Cos that inductor is 13x13x8mm, would like something smaller if it's possible :)

--- End quote ---
With lower inductance this converter may stay a bit longer in PFM mode, but I do not think that you get a huge power savings from it.
For light load scenario i usually tend to choose higher inductor values (so 4.7uH or perhaps even more) to get better regulation and lower ripple current.

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