In one of my earlier posts I mentioned that I got brownouts and resets on the mcu whenever I powered up the RFM when using the MCP1703 LDO. I think I now have a solution that may be worthy of mentioning here in this thread. It's not so much about which LDO to pick to improve transient response, but (for me, in this particular project) a way around it.
My project uses a small, 100mAh (or 150mAh) LiPo battery because I want to keep the overall size small and this is about as big a battery that will fit in my enclosure. I always preferred running with 1S as it just simplifies re-charging. Since I was getting quite frustrated with the poor transient performance of the 1703 and didn't want to add big capacitors on an already small pcb, I investigated the option of running everything directly off the battery. My critical components are: RTC, ATmega328p, RFM (12B or 69) and a DS18B20 temperature sensor. Except for the radio module, all other components can run on as high as 5.5V. The RFM datasheets say that the maximum Vcc voltage is 3.6V for the RFM69's and 3.8V for the RFM12B's. They would be out of spec if I were to run them on 4.2V (a fully charged LiPo).
This is where I was stuck for a while until I tested with putting a diode in series after the fet that switches the power to the radio. A simple 1N4001 has a voltage drop of about 0.7V and if my LiPo is fully charged, the diode will drop it down to a safe 3.5V for the RFM. I noticed that the voltage drop changes somewhat when there's more or less current, but just 1x diode is enough for this application.
Losing the LDO made several improvements for this project:
- less current drain on the battery as the LDO is not taking up any power anymore
- more space on the circuit board as the LDO and its surrounding caps are gone (although I do now have to add the diode, DO-214 footprint)
- made the design somewhat simpler and cheaper
- improves power stability as the LDO no longer causes a dip whenever a big(ish) load is connected
- longer battery life as the LDO needs to overcome the dropout voltage (according to the datasheet a 3.3V 1703 is out of spec when fed with less than 4V on the input and supplying the maximum 250mA output). Not that I want to, but I can run my LiPo now down to 3.3V and still get the radio to work.
- no longer a restriction of 250mA current draw
I have this now in working operation with the RFM12B's and expect to upgrade to RFM69CW once I have redesigned the pcb's.