Author Topic: Cheapest way to do 2 cells (alkaline) and a 5V USB jack both to 3.3V output  (Read 2064 times)

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Offline alank2Topic starter

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The idea would be, if plugged into USB, the power comes from USB, but then the 5V has to be regulated down to 3.3V.

Also need to support 2 alkaline cells (1.6V to 3.2V) that boosts to 3.3V.

I've been using some Shottky diodes to combine 5V USB and 3 alkaline cells (2.4V to 4.8V) so the output natually follows the highest voltage (USB if plugged in), but I want to do a dual power source USB and 2 cells that both go to 3.3V.

Should I put the 2 cells through a switcher step up to 3.3V and the USB to a linear 3.3V step down and then try to combine them with ?

I saw some step up / step down ones, but they were pricy if I was looking at something with leads on it like a soic-8...
 

Offline alank2Topic starter

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Current needs 200mA max.
 

Offline schmitt trigger

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Linear Tech have some unique buck/boost ICs which include the power Mosfet.
I would start from there.
And feed it from either USB or battery via Oring diodes.
 

Offline alank2Topic starter

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I saw one of them, but as the voltage goes lower, so does its current capability.  Do you have a part number I should look at?  Any other suggestions even if it isn't a single part solution?
 

Offline mariush

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See SEPIC regulators.
For example  LM2621 : http://www.ti.com/lit/ds/symlink/lm2621.pdf
Page 14 has example circuit for 1.25v to 14v -> 3.3v 500mA max and you have explanations on how to pick values if you only need 200mA (for increased efficiency and other things)

 

Offline alank2Topic starter

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So even if the input is too high, it will still have a 3.3V regulated output?  I was reading through the datasheet, but it talks mostly about step up and I don't see it saying much about stepping down.
 

Offline Rolo

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Take a look at this module, I think it does what you're asking for: https://www.pololu.com/product/2563
 

Offline mariush

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Yes, that's what SEPIC means... see https://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter

SEPIC converters are noticeable by the presence of two separate inductors, or by an inductor with two windings.


Something else you should consider...  if you don't care about efficiency when it comes to usb power or heat, you could just use a cheap LDO to get the voltage down to something below 3.3v but above what you'd typically have with batteries, and then simply use a cheaper step-up regulator.
For example, 2.8v , 3.0v , 3.3v are standard voltages and there's tens of thousand LDOs out there with these preset outputs ... if you use two rechargeable AA batteries you'd have up to 2.7v (1.35v x 2) .. alkaline batteries are up to around 1.65v, so 3.2v max
For example, MIC5504-3.3 is 10-11 cents each and can do up to 300mA : https://www.digikey.com/product-detail/en/microchip-technology/MIC5504-3.3YM5-TR/576-4764-1-ND/4864028
Same for MIC5501-3 which outputs 3v at up to 300mA : https://www.digikey.com/product-detail/en/microchip-technology/MIC5501-3.0YM5-TR/576-4905-1-ND/5277876
Here's a whole lot of them : https://www.digikey.com/short/pp82cz

So once you know you have 3.3v or less no matter the source, you can focus on using a step-up / boost regulator only ... and your parameters are less than 3.3v input, probably minimum 1.8v or thereabouts, 3.3v output , around 200mA of current ...
Here's a starting point ... all switching regulators that can start with less than 2v and output at least 250mA of current, sorted by price @ 25pcs : https://www.digikey.com/short/pp82tw

So you have for example MCP1640 at 50 cents @ 25pcs https://www.digikey.com/product-detail/en/microchip-technology/MCP1640T-I-CHY/MCP1640T-I-CHYCT-ND/2258621
which can do 0.8v ..5.5v step-up to 5.5v 350mA , sync rectifier so no need for diodes...
•   Up to 96% Typical Efficiency
•  800 mA Typical Peak Input Current Limit:
-IOUT > 100 mA @ 1.2V VIN, 3.3V VOUT
-IOUT > 350 mA @ 2.4V VIN, 3.3V VOUT
-IOUT > 350 mA @ 3.3V VIN, 5.0V VOUT

Or TPS61322 , 75 cents @ 10pcs, 58 cents @ 100pcs : https://www.digikey.com/product-detail/en/texas-instruments/TPS613221ADBVR/296-50502-1-ND/9685641
0.9v to 5.5v in, it will do 200mA from 1.5v and up

So basically the overall cost would be less than 1$ but the downside is that the LDO will dissipate some heat taking 5v from USB down to 3.3v or less... at around 250mA, we're talking (5v-3.3v) x0.25a= 0.425 A .. it's not a big amount.




 

Offline David Hess

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A buck-boost, flyback, or SEPIC regulator is the way to go.  SEPIC and flyback regulators can use any controller intended for step-up operation which is convenient because the output switch can be an NPN bipolar or n-channel MOSFET.

The major complication is getting full enhancement of a power MOSFET if used from a low input voltage source.  There are various ways this is handled.

Something like the buck-boost LTC3536 or SEPIC LT8494 or LT1308 would be suitable.
 

Offline alank2Topic starter

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Thank you guys for all the directions to look at; it will have a microSD card too so there might be some inrush from that being inserted too.
 

Offline bson

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Thank you guys for all the directions to look at; it will have a microSD card too so there might be some inrush from that being inserted too.
Make sure to give the SD card a filter cap on Vcc.  I like to use 22µF, just something a little bigger than merely decoupling.
 


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