I bought a sim900 (the chip itself, not a module) and have gotten stuck trying to figure out how to power it. I hope to power it using a pin from my RBBB arduino clone, which outputs 5 volts and though sources differ, either 250mA or 300mA. It uses a L4931 power regulator, if that makes a difference.
So far, the most helpful website has been this one which has schematics for an usb powered 500mA power circuit.
I don't understand what is happening on this diagram. For one thing, the part I was most worried about, the capacitor, seems not to be listed. The description of the diagram states that "A low ESR tantalum capacitor is usually used. The value for the capacitor should be more than 470uF."
It also says that "When there was no on/off pin in the LDO or DC-DC IC, the customer can follow nether reference schematic to control the VBAT on/off." which seems to imply this doesn't actually show all of what goes into the VBAT and there's more to the circuit outside the VBAT_IN.
I'm also confused as to what the MCU_CTRL is. The pinout here: simcom(dot)us/act_admin/supportfile/SIM900_HD_V1.01%28091226%29.pdf#page=16 of the sim900 shows no such pin.
There is also a different diagram from this site that looks completely different.
If somebody could explain which diagram is right to use, and what the parts do in that diagram, that would be fantastic!
[Edit: after looking at the parts more closely, it seems that figure 5 is a better fit, despite figure 3 specifically mentioning usb power. I am still confused on that one what the purpose is of the capacitors leading to ground, and the inductors. Also, if this is a LDO, then the diagram on the 1st website a litter higher up might be a good fit. That site mentioned that it needed a 2 amp power in to the regulator. Is that what the capacitors are for?]