Hi,
Design question. I have come across SIM7020E LTE modules. They are 3.3V and have a nicely low power mode (PSM; Typical:3.4uA!).
Noting that the moteino LDO is 250mA max this module should work off the 3.3V pin as "LTE transmit" is listed as 134mA. This assumes running off a LIPO battery.
However the datasheet has "Module power peak current in NB emission" as 500mA.
So question is how would you design to power that? Specify an LDO for the module that can deliver 500mA? or risk it? or is it possible to add some large capacitors between the 3.3V pin for the Moteino LDO and the VCC of the module so cover those peaks? What is the implication of doing that?
There is a section in the datasheet;
QuoteNote: If the power supply for VBAT pins can support up to500mA, using a total of more than 100uF capacitors is recommended, or else users must using a total of 300uF capacitors typically, in order to avoid the voltage drop. The module power peak current depends on the total capacitance. Using a total of 1000uF capacitors in the test that will reduce the peak current to 320mA.
Not quite sure what that means, bar adding 1000uF drops peaks to 320mA which is still not quite enough. 2000uF?
I am clearly guessing here.
Hardware datasheet;
http://www.mt-system.ru/sites/default/files/docs/simcom/docs/sim7020e/sim7020_hardware_design_v1.02.pdf
I would recommend a separate VR that can handle a 500mA load and at least 1000uF cap for this. Or, if you have a higher voltage source, use a LiPo battery, kept charged from the higher voltage, but easily able to supply the surge needed by this device. The Moteino VR is not designed to handle this load.
Tom
It depends on whether you need to go through the LDO.
From what I recall, some (if not most - I am more familiar wit the older GPRS generation modules like SIM8xx,SIM9xx) of these modules can take up to 4.2V+ which is compatible with powering straight from LiPo's, so in that case some local caps as recommended by the DS, on the VCC of the LTE module would be enough.
The Moteino's LDOs are 250mA rated, and are specifically intended for the Moteino+RFM module + any other small peripherals and typical CPI/I2C/sensors that one might add, and not for power hungry external devices.
If the spikes are very short - I think usually a few ms (also from recollection of GRPS modules) - then loading the VCC with the right capacitance to assist the LDO might do the trick, but you have to be careful and not guess your way around this.
You will have to scope this out and see what happens in that case, look at the drops and also ensure the LDO is doing OK.
This model takes 3.3V only apparently - so not able to power direct from LIPO.
I think the peaks to 500mA are short, as none of the other data seems to suggest going over 134mA.
How would I work out the capacitance needed?