The data sheet says that the maximum current is 50mA into AC1/AC2.
When I look at solar cells (from Digikey) it looks like the biggest solar cell that has a current <=50mA is 229mW
I guess my question is that it looks like there is a maximum IV depending with I fixed at 50mA... am I reading that right?
If you could find a solar cell that did 50mA @ 19V that which is about 1 Watt... but that solar cell doesnt seem to exist in the real world... and it looks like the biggest practical cell is around just a bit over 200mW.
The input rectifier diodes internal to the LTC3331 have a 50mA limit. Additionally, there is a clamp that engages at ~19V with a 25mA limit; so, if your input voltage is >19V, you need to be limited to 25mA.
I'm not understanding what the problem is. Are you not able to find a panel powerful enough to supply your load?
If you can't get enough power out of the LTC3331, you may want to consider a different part (I can help you choose). LTC3331 is meant for micro-power applications, so if you're bumping into the limits of the part then it may not be the best choice.
What do you need for your panel/load?
I would like to use a panel in the 1-2watt range... something like 6v @180mA
The way that I read the datasheet there is a limit of 50mA into AC1/AC2 ... I guess I thought that meant that my panel sourced too much current. On page 2 of the datasheet it says "Iac1/Iac2 max = 50mA"
Take a look at the LTC3331 block diagram. The AC1/AC2 inputs just go to a rectifier which outputs to the VIN rail. The 50mA limit is enforced as to not damage the internal rectifier diodes. For a solar input, you can bypass the rectifier and input directly into the VIN rail but, if you have another input going into AC1/AC2, you still need to be careful not to force a voltage across the rectifier diodes. So, a solution might be to put your own (more powerful diode) between your source and VIN, or you could use a resistor -- it depends on what is more efficient in your application.
Just be aware that the buck switch peak current is a typical 250mA. You won't damage the IC by bumping up against this, but you still may end up operating slightly below the max power potential of your solar panel. My guess is that this would be a negligible loss, though. The amount of input current, of course, depends on your input voltage, output voltage, and load current.
OK. (hopefully I am not too annoying... but I appreciate your answers).
If I restate what you said I can.
Hook Solar & USB in a diode wired or into VIN... with diodes between them to prevent driving current the wrong way into them.
Or... I can have either the USB or the Solar into AC1/2... but then I need to limit the current to 50mA with a resistor. In this configuration what I don't get is how big to make the resistor? (im sure when you answer this question Ill say "oh obvious why didnt I see that" ... must be set by the ULVO and the input resistance? But im not seeing it.
Thank you for your patience... this is a super cool chip.
The other thing that seem strange to me is that the current limit into AC1/AC2 is 25mA... almost any solar cell you can think of that is more than 5v is going to have a Impp > 25mA... so i suppose that it seem that as you can tolerate unto 19v... it is surprising that the limit is so low.
What am I missing?