I'm currently designing a PCB for a caving robot, where PCB space is rather limited. One thing taking quite some space are the input and output capacitors of the 12 to 5VV DC-DC converter.
I basically just used the values from the typical application from the "core" of the regulator, the XL4015 (datasheet) :
For now, I used just reused part numbers I already used elsewhere (to reduce cost),but I'm intending to use just one 220/330uF capacitor instead of the 2 or 3 I initially planed to save space.
I was thinking to go for tantalum capacitors (as far as I understand, they are the smallest for given capacity and voltage): does that seem OK? If so, what voltage ratings should I use? (the lower the voltage, the smaller the capacitor, but to what point can I push without risking failure?)
Additional information :
- the "12V" is non regulated voltage from a 3S Lipo (9 to 12.6V), and also connects to 16 small (<1A) motor controllers
- the 5V will mainly power a Raspberry Pi and the linear voltage regulator providing the 3.3V for the STM32F7, and maybe a few sensors later on, maybe a few power LEDs with PWM fading (those will probably run from the 12V directly, but to be confirmed). I expect usually 2-3A, with peaks a bit higher
NB : in the typical application notes, they use :
- 50V for the input (but input voltage is up to 38V) : I suppose it is completely overkill when I have only a 3S lipo as input
- 25V for output : I realized they used this same value for 5V and for 12V typical application : is there any reason to keep it that high? If not, how low can I go? 6.3V? 10V?
Thanks a lot in advance
EDIT : As asked, some aspects about safety requirements :
- failure in the DC/DC converter (whatever the reason), leads to about 90% risk of loosing the robot (one might be happy and it is still near enough to retrieve it by hand, but even so it might get damage by the fall). Durring testing/prototyping, the robot is accessible, so no risk of loosing it, small damage if falling.
- the components for one robot will be around 400€ (nb : I'm not planning to sell, just to put it open hardware if it works)
- in the cave, there is nothing that might catch fire, so the only risk is when testing the robot, or when handling the robot by hand
- no risk of the robot harming humans in case of failure (excepted the flame/explosion of the tantalum itself if the user is still holding the robot at the start/end of mission
- I guess the risk of loosing the robot for any mission is something like 5 or 10%