@nils I believe this would really only be necessary for 2G though which is 1800mA peak, 3G is 800mA peak and LTE is even less at 490mA peak. We are using the recommended bulk capacitance by u-blox at the input of the modems, which should handle these peak currents without browning out the supply. We’ll run some tests to double check if under continuous TX/RX, these on-board caps are sufficient in handling the peak currents or if they are able to deplete the bulk caps and drag the VUSB input down below the minimum input voltage set. If that happens, the PMIC will switch over to the BAT input for a backup supply. So if no battery is present the modem would receive a brief power brownout and the PMIC would then switch back to the VUSB/VIN input.
A good solution to avoid this potential issue if you don’t want to use a battery, is to supply the Li+ input with a regulated 4.2V, and disable charging with PMIC().disableCharging(); You can also add a USB connection in a data only configuration, by not connecting the VUSB pin, to avoid the possibility of back-feeding your 4.2V supply when USB is plugged in.
The reason the input current limit is set to 900mA is because there is not enough heat-sinking capability inherently on the boards that use this PMIC. I believe you can also only go up to something like 1200/1300mA as well since there is a HW limiting resistor option stuffed on the board that will ultimately cap the upper end.