So, I had a look at some previous designs. There are some constraints:
- 56V input is quite high
- you need high efficiency at light load (note, many data sheets only talk about shutdown current rather than operating current)
Given those constraints, I would suggest the TI LMR16006 lineup. They aren’t the best and brightest, but they are small and cheerful.
To run Bluetooth, you probably need 100 - 200 mA peak current for the system. Therefore you want a LDO (linear Low DropOut) regulator with 250 - 500 mA rating and low quiescent current. I’ve used the MCP1700T-33 (fixed 3.3V output, 4uA quiescent current) and it seems to work.
The LDO needs some headroom to operate. The MCP1700T datasheet lists a maximum output voltage tolerance of +3% (ie 3.40V) and a dropout voltage of 350mV. That means we need at least 3.75V from our buck regulator.
The buck regulator has tolerances too. I would allow for 3% reference accuracy, 2x 1% feedback resistors and 5% margin. This sums to 10%, so the target buck output voltage should be 3.75 / (100% - 90%) = 4.16V. Say 4.2V for safety.
For 60mA @ 3.3V (198mW) output, we need 60mA @ 4.2V (250mW) from the buck regulator. Expected buck regulator efficiency is around 65% (using Webench from TI), which means buck input power is around 250mW/65% = 384mW, or 7mA. Overall losses are around 186mW, and overall efficiency is around 50%.
BT transmit peaks are short and shouldn’t cause much temperature rise.
While I’m pontificating:
- try to have your BT module and microcontroller sleep as much as possible to reduce average current
- I suggest charging cells to 4.1V to improve cycle life (will reduce energy strorage by 10 - 15%) and slightly reduce fire risk
- make sure that your BMS will be safe even if the BT module starts doing weird stuff (or locks up)
- pleas be careful when handling raw battery packs. Safety glasses suggested (I almost got myself in the eye with a spark last year). Also, the heat shrink over those cells is really thin and easy to scrape through.