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LTC4425 - some questions

pityocamptes

Jul 26, 2012
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92_circuit_1.jpg



Ok, so let me get this right. The PROG pin with a resistor at 500 ohms, will have a max current outout of 2A. Now if it has a max current limit of 3A and my power supply provides 3 amps MAX, could I change the PROG resistor to 333 ohms for 3 Amps max?

Also, the Vin/PFI/PFI-RET split resistor group should be set at 2.5v, which is the power failure voltage correct? Since SEL = VIN, then the SuperCaps should be charging at High = 2.7 v per cap? Also, since no voltage out divider exists, will the circuit above basically push out Vin = Vout, as long as Vin does not fall below 2.5v and trigger a power failure, super cap backup? Finally, on PFO, what exactly does the 470K resisitor do, and where does this pin attach to? What exactly is the Vio on this pin - does that attach to Vin, or ground, or does this resistor just float, not attached to anything? Thanks!

Also, where does the current monitor leg go to, how would this be used? A simple current/volt meter?

My Li-Ion power supply would actually be a non regulated 3.5 volt min - 5 volt max, 3 amp max, dc supply.
 
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Harald Kapp

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The LTC4425 is rated for a typical charge curent of 2A. The value 3A in the datasheet is the peak current. It is generally not advisable to operate a component at its limit.

If you want to try anyway: The datasheet states "...the constant-current amplifier is in control and servos the PROG pin voltage to 1V. The current through the PROG resistor gets multiplied by approximately 1000" so 333 Ohm should be equivalent to 3A output current.
I'm almost sure the chip will not survive for long.
 

pityocamptes

Jul 26, 2012
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The LTC4425 is rated for a typical charge curent of 2A. The value 3A in the datasheet is the peak current. It is generally not advisable to operate a component at its limit.

If you want to try anyway: The datasheet states "...the constant-current amplifier is in control and servos the PROG pin voltage to 1V. The current through the PROG resistor gets multiplied by approximately 1000" so 333 Ohm should be equivalent to 3A output current.
I'm almost sure the chip will not survive for long.

What about this (4 amp)?

ISC Charger Short-Circuit Current Limit PROG Pin Shorted to GND, FB = 0 2 3 4 A
 

Harald Kapp

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As it says "Short-Circuit-limit". This is no standard operation.
 

pityocamptes

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Also how difficult would it be to add a charging and charged leds? Maybe a green led that shows when the caps are charged and when it isn't the green turns off and a red one shows charging, and vice versa?
 

Harald Kapp

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PFO = Power Fail Output. You don't have to connect it to anything if you don't need the PFO indication.
There are no provisions for indicator LEDs as qou request.

As to the other questions I suggest you read the datasheet thoroughly. I'm sorry, I have not worked with the LTC4425 before.
 

pityocamptes

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PFO = Power Fail Output. You don't have to connect it to anything if you don't need the PFO indication.
There are no provisions for indicator LEDs as qou request.

As to the other questions I suggest you read the datasheet thoroughly. I'm sorry, I have not worked with the LTC4425 before.

Thanks. Is there any place or way to add a voltage sensing circuit for an led? Really wanted an indication if it was charging, charged, etc.
 

Harald Kapp

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To see if it is charging add a current sense resistor between charger and battery. Amplify the sense signal and drive an LED. This will give you an indication whether the charger is charging or not.
You could also add a comparatorparallel to the output which can give you an indication whther the capacitor is charged or not. If you want a more detailed info about the charge status you could use an LM3914 bar graph indicator circuit instead of a simple comparator. The linked example will have to be adapted to the voltage levels of your circuit.
 

pityocamptes

Jul 26, 2012
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To see if it is charging add a current sense resistor between charger and battery. Amplify the sense signal and drive an LED. This will give you an indication whether the charger is charging or not.
You could also add a comparatorparallel to the output which can give you an indication whther the capacitor is charged or not. If you want a more detailed info about the charge status you could use an LM3914 bar graph indicator circuit instead of a simple comparator. The linked example will have to be adapted to the voltage levels of your circuit.

Thanks! One more question. On linear techs site you can purchase the ltc4425 directly from them. I know what the difference is between the MSOP and DFN packages. However, why the difference in part numbers???

LTC4425EDD#PBF Linear SuperCap Charger with Current-Limited Ideal Diode and V/I Monitor DFN 12 $3.22 $2.73
LTC4425EMSE#PBF Linear SuperCap Charger with Current-Limited Ideal Diode and V/I Monitor MSOP 12 $3.36 $2.85
LTC4425IDD#PBF Linear SuperCap Charger with Current-Limited Ideal Diode and V/I Monitor DFN 12 $3.71 $3.16
LTC4425IMSE#PBF Linear SuperCap Charger with Current-Limited Ideal Diode and V/I Monitor


http://www.linear.com/purchase/LTC4425
 

pityocamptes

Jul 26, 2012
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To see if it is charging add a current sense resistor between charger and battery. Amplify the sense signal and drive an LED. This will give you an indication whether the charger is charging or not.
You could also add a comparatorparallel to the output which can give you an indication whther the capacitor is charged or not. If you want a more detailed info about the charge status you could use an LM3914 bar graph indicator circuit instead of a simple comparator. The linked example will have to be adapted to the voltage levels of your circuit.


Thanks! Also found this for the LM3914 that will work for me. Do you know if the LM3914 will operate at a 2.5 volts? The data sheet said it will operate below a 3 vin, but 2.5 will probably be the absolute lowest limit for my circuit? Nice ideas over here:

http://www.teachersyndicate.com/documents/oct_2010/Circuits for the Hobbyist.pdf
 

Harald Kapp

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The part number reflects the case style and package options.
You can try the LM3914 at 2.5V, some specimens may work, others not. It is outside the guaranteed operating range so noone can give you a guarantee. Besides, depending on the color of the LEDs you will need more than 2.5V anyway.
I recommend not to rsik anything and use a suitable supply voltage.
 

pityocamptes

Jul 26, 2012
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Linear tech got back to me and said that if you connect an LED in series between the PFO and resistor that you can use that as a charge light (on when charging, off when not). What then does the other side of the resistor connect to? The voltage input rail or ground? Thanks!
 
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