# Need some help with LTC4425

Discussion in 'General Electronics Discussion' started by pityocamptes, Sep 6, 2012.

1. ### pityocamptes

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0
Jul 26, 2012
I'm not sure if this is the chip that I actually need, and if so, what the program values I would need to meet my goals. Here is what I am trying to do. I would like to auto charge two supercaps (in series) from an input voltage of between 2.7-5.5 volts (should be stable at about 4.5-5.0 volts at 3amps max) - the super caps have a max voltage of 2.7V. I would like to use the maximum amount of amps from the power source to the supercaps. Anyways, I would like to build a circuit that charges the super caps, but also allows me to power a small device. Basically, use the small device from the input voltage, but when the input voltage is terminated, the supercaps would kick in (sort of like a backup batt system). I was looking at the LTC4425, but I am not sure if this would work for what I need. If so, could someone help me figure out how I would "program" it for my needs? I was looking at the datasheet, and it would seem that if the vout does not equal vin then I won't get the max current? Maybe I am reading this wrong. If this is not what I need, can someone suggest something simple that may work? Thanks again!

http://cds.linear.com/docs/Datasheet/4425f.pdf

2. ### BobK

7,682
1,689
Jan 5, 2010
It is all in the datasheet. You program the voltage to charge to with this equation:

VOUT = VFB • (1 + RFB1/RFB2)
where VFB is 1.2V.

i.e. divide Vout by 1.2 to get the ration to make the Rfb1 Rfb2 divider.

Example: To charge to 3.6V use a divider that will divide the voltage by 3, like 2M and 1M.

And the charging current with this equation:

RPROG = 1000 • (1V/ICHRG)

i.e. the resistor is 1K / <charge current in amps>

Example: to charge a 2 amps, use 1K / 2 = 500 Ohm.

Bob

Last edited: Sep 6, 2012
3. ### pityocamptes

79
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Jul 26, 2012

OK, thanks. I will look at it again. The sheet had several different schematics and I was not sure which one to go by. Perhaps you can help on that? Kind of new to this whole thing. Thanks again.

4. ### BobK

7,682
1,689
Jan 5, 2010
Explain more about what you are trying to do. The purpose of this chip is to charge up a couple of supercapacitors so that they can provide more current in a short burst than the battery or power supply can supply. Is this what you want? If so, choose the sample application circuit closest to what you want and adjust the 3 resistors to program the voltage and current you want.

Looking at your OP, you said you wanted to charge supercapacitors from a 2.7 to 5.5V (or is it 4.5 to 5V?). This chip will not charge to a higher voltage than the input if that is what you are thinking.

What is your input source, (USB port, maybe?) And what is it providing power for?

Bob

5. ### BobK

7,682
1,689
Jan 5, 2010
Another question, what is your experience? This chip is a surface mount chip with a thermal pad that must be soldered to a ground plane of a PC board, which pretty much means that it must be soldered by reflowing. If you did not understand any of that, you probably cannot use this chip.

Bob

Last edited: Sep 6, 2012
6. ### pityocamptes

79
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Jul 26, 2012

Thanks. No I realize that. I have done reflowing before. I am pretty good at soldering, and have the equipment, I just suck at trying to figure out schematics that are not set in stone for me to follow. Thanks again!

7. ### pityocamptes

79
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Jul 26, 2012
Bump? For help?