I'm trying to get power supplied to a 3g cellular board. The board manufacturer, Adafruit, changed the requirements on me after I started putting the board together, to say that it requires a li-po battery, but this is going into a barn where it won't be attended. Those batteries are a known fire hazard, so I'm trying to do a non-li-po supply. I have about 600Ah of solar-powered 12v batteries available, which I have to step down to the 4v the 3g chip wants.
The chip manufacturer says you should be able to do this with a LM2596 switching regulator, but recommends a 100µf tantalum capacitor or greater. Not having any tantalum caps handy, but having several 1.5f super capacitors around, I tried to use that, but the chip is behaving as though the voltage is dropping too low, and restarting.
This 3g chip has a slightly odd power requirement:
Diagrams are from the SIM hardware design document for this chip: http://www.adafruit.com/datasheets/SIM5320_Hardware%20Design_V1.07.pdf My LM2596 matches this diagram, except I don't currently have the TVS diode and fuse on the input side, and don't have the FB101 on the output. Instead I have a thermistor then the supercap.
When I plug this into a calculator for capacitor discharge here, I get values that say I should get a pretty minuscule voltage drop, but instead the board is resetting from voltage drop. Is this ripple? Is it the ESR of the supercap? Is this why SIM recommends tantalum caps? I'm assuming the switching supply produces more or less clean enough voltage, the thermistor is properly getting out of the way after the supercap initially charges up (seems to in my testing), and the supercap is more than large enough to deal with this surge current. So where am I going wrong. Also, which tools would I use to diagnose this? Thanks for any thoughts!
The chip manufacturer says you should be able to do this with a LM2596 switching regulator, but recommends a 100µf tantalum capacitor or greater. Not having any tantalum caps handy, but having several 1.5f super capacitors around, I tried to use that, but the chip is behaving as though the voltage is dropping too low, and restarting.
This 3g chip has a slightly odd power requirement:
Diagrams are from the SIM hardware design document for this chip: http://www.adafruit.com/datasheets/SIM5320_Hardware%20Design_V1.07.pdf My LM2596 matches this diagram, except I don't currently have the TVS diode and fuse on the input side, and don't have the FB101 on the output. Instead I have a thermistor then the supercap.
When I plug this into a calculator for capacitor discharge here, I get values that say I should get a pretty minuscule voltage drop, but instead the board is resetting from voltage drop. Is this ripple? Is it the ESR of the supercap? Is this why SIM recommends tantalum caps? I'm assuming the switching supply produces more or less clean enough voltage, the thermistor is properly getting out of the way after the supercap initially charges up (seems to in my testing), and the supercap is more than large enough to deal with this surge current. So where am I going wrong. Also, which tools would I use to diagnose this? Thanks for any thoughts!