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Author Topic: LM2575 7.4V to 5V (i don't understand the circuit diagram)  (Read 454 times)
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Hello arduino community! I'm an arduino owner for 4 months.

Well, i'll be going for a holiday next sunday (yay!=) and i told myself to make a portable iphone charger.

So, i made some research for a week and i learned something about iphone 5 charging system;

iphone usb has 2 power pins and 2 data pins. these are Vcc, D+, D-, Gnd.

Generally usb devices doesn't really use D+ and D- while charging. But iphone uses those pins to determine how much power to draw from the source.

For fast charging : D+ = 2.2V D-=2.0V
For portable charge : D+ = 2.0V  D- = 2.0V

well, i think i could handle fast charging with my 7.4V 1500mAh Li-ion battery. So i bought some LM2575 voltage regulator.

I wasn't very into using advanced circuits (like capacitors, voltage dividers etc. fancy stuff smiley) so i stuck at the diagram.


i can only see that i have to put 100uF and 330uF capacitors , 1 diode and something looks like resistor.

I don't really understand those things, and i don't even have my diodes (im using a chinese website and shipping takes time)
is there any way to simplify this?

Also, i don't have 330 uF capacitor but i have plenty of other capacitors  (100uF, 440uF etc..) does this works like resistors (like series = R1 + R2 , parralel  1/Rall = 1/R1 + 1/R2)

if anything is unclear / or something is missing, just ask! that's all i know about it.

Fun Stuff : Electronics vs Me

Uh oh. Ok then.
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Its a switch-mode buck regulator circuit.  L1 is an inductor, crucial to whole thing - until you understand inductors
there's no point trying to figure this one out.

Such circuits are very fussy about components, you cannot just buy any capacitor for instance as series
resistance and self-resonant frequency matter.
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The LM2575 is one of the least complicated (external component wise) of the switching regulators smiley

The thing you thought was a resistor is an inductor, careful picking the right one as a 3a inductor 300uh  is not the same as a 300uh  0.3 watts lol

Have you found the resistor value to enable high current charging?
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The Schottky diode may be critical too.   Where I work we somehow got the wrong diode on a board with a similar circuit and the voltage regulator chip got very hot and the voltage regulation was unreliable.

I suggest you carefully study the LM2575 datasheet.
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The Schottky diode may be critical too.   Where I work we somehow got the wrong diode on a board with a similar circuit and the voltage regulator chip got very hot and the voltage regulation was unreliable.

I suggest you carefully study the LM2575 datasheet.

The schottky is important,  due it's fast switching capabilities compared to regular diodes.
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Wow. Things are much more complicated than i expected. So, what do you guys reccomend me? I have a friend that knows a lot about electronics. He's designing boards for arduino, repairing machines etc. Maybe i should visit him and ask some parts from  him.
Also, i tried connecting pin 2 to pin 4 with 10k potentiometer. at lowest setting it gave me 5.58V which is way too much than i need. But it won't be as solid as the circuit at datasheet.

I'm still looking for advices.. Thanks !
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The LM2575 is one of the least complicated (external component wise) of the switching regulators smiley

The thing you thought was a resistor is an inductor, careful picking the right one as a 3a inductor 300uh  is not the same as a 300uh  0.3 watts lol

Have you found the resistor value to enable high current charging?

Well, not really. I was thinking about using a voltage divider for that.

I don't know how much iphone draws, so i can't connect it like connecting an LED.i know resistance of an LED, but i can't tell the same for the iphone.
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To better understand the intricacies of charging Apple products: https://learn.adafruit.com/minty-boost/icharging
To try to clarify-
Quote
I was thinking about using a voltage divider for that.
Yes,  voltage dividers on the D+ and D-  are required for charging.
Quote
Also, i tried connecting pin 2 to pin 4 with 10k potentiometer.
No, you must use the proper inductor if you want the chip to work as designed.

The LM2575 Is fairly easy to use as long as you use the right inductor (L1)- NOT a resistor. A Schottky diode is important as well (fast switching with high current), but selecting the right inductor is crucial to the stability of the circuit and accuracy of the voltage regulation. The datasheet for the LM2575 should include a fairly detailed explanation of exactly what part is required.  Read that datasheet! Power inductors aren't like simple resistors where any which one will do, and I would think it unlikely that you're friend would stock them, (unless of course he has built this circuit before.) The three main variables associated with selecting the inductor are the inductance, current capability, and resistance, the most important of which being the inductance value and current capability. The resistance corresponds to http://en.wikipedia.org/wiki/Q_factor of the inductor and determines how efficient and accurate the circuit is.
Quote
Things are much more complicated than i expected
  Since it is a switching regulator, it is not as simple as say-a 7805.  But it doesn't need to be overly complicated as long as you pay attention to the important bits in the datasheet.
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I recommend you use the charger that came with the phone !!
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I recommend you use the charger that came with the phone !!
A thousand times this post!

Another important topic that has been missed in this discussion is that the LM2575 is a voltage regulator - which is only part of  the circuitry required to properly charge an iPhone battery. You also need a current regulator that changes it's output as the battery charges.

You're headed down a path the will most likely end, best case, with the destruction of the battery. Worst case is that you destroy the phone as well in the process.
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Another important topic that has been missed in this discussion is that the LM2575 is a voltage regulator - which is only part of  the circuitry required to properly charge an iPhone battery. You also need a current regulator that changes it's output as the battery charges.

You're headed down a path the will most likely end, best case, with the destruction of the battery. Worst case is that you destroy the phone as well in the process.

No, he specified a charger for the phone, not an iPhone battery.  A voltage regulator is exactly what is required since the charge controller is already part of the phone.  Of course, these circuits cost approximately $2.50 on eBay, ready-built, but you have to realise this well in advance due to the postal times.

I recommend you use the charger that came with the phone !!

Excellent idea - except perhaps that he actually wants to use his present Li-Ion model car/ boat/ plane/ helicopter battery as a "power bank" for the phone whilst not near mains outlets.
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Nope this is a good way, he's not charging the battery, the internal circuitry of the phone deals with that, charge your phone via 5v.
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F.Y.I.  about $3.00 on e-bay.   $3.25 if you want an LED to display voltage.

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There is a certain satisfaction that comes from DIY Projects especially in the face of all the nay-sayers; there is also great potential to destroy your phone, burn your house down, and die a fiery death.  Tread carefully. The benefit of building the circuit yourself is that you not limited to  a set form factor and you can add it to a PCB design. In this case, it probably would have been easier to go with a manufactured part.  Live and Learn.
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I needed the same part.  have a home brew arduino based PLC, 4 relays, 4 A/I you know, bring the pins out to things.

but, to power the relays and the remote sensors I wanted to have one power feed and then output from the one board.   so I put on the 2576 5v chip to get the higher power.

added the rectification circuit to I could power the outputs with 24VAC and have 5v on board.

when you need it, these are simple chips.  just follow the data sheet and you should be happy.

but, when you order the chip, make sure you order all the bits for the whole circuit !

Dave
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