I am building out a board to support our developer edition electric imps using a polymer li-ion battery. I am using a LDO VREG which performs pretty well at low current but the dropoff at .5A will be troublesome. So, I am looking to put a decent sized CAP on the board to try and prevent current dropoff when WIFI spikes.
So, my question is
- Are there internal caps so that I do not need to worry about adding a cap myself?
- Can the in-house engineers make any recommendations on cap characteristics given their understanding of the wifi operation at 3.3V.
There is no large cap inside imp, it just would not fit. So if you need to save energy energy in the cap to overcome short power spike needs, you need to add this externally. In many mobile dongles this capacitor is the largest part of the design, as it has to supply 2A at short times. 400mA is not that bad, it but you need to know the maximum time this current is taken to calculate the capacitor value needed.
Hopefull the imp team will make this calculation and offer some ballpark values for this cap.
Ideally your supply should be sized to be able to supply 400mA all day (well, maybe not thermally but for milliseconds). You should absolutely be using an LDO that is rated to 400mA at the Vin-Vout differential you’re working with. Software changes and ambient network conditions can cause different current profiles due to different TX duty cycles.
Do not expect a cap to “fill in” for an undersized regulator. Note that the Vin-Vout differential is very important, as the current rating of an LDO is usually determined by the Rds(on) of the pass FET internally, hence the bigger Vin-Vout you have, the more current you can support.