I have an RPi that I want to give enough juice to auto shutdown when the power goes down. For this I'll use 2 supercaps of 6F (2.7V) in series, giving me plenty of time. Now, I was wondering if I could charge these supercaps on the same 5V source that I connect to the RPi. The powersupply gives maximum 2.4A, but I want to limit the current into the supercaps so there is enough for the RPi and have a low voltage drop of around
0.1-0.2V. I thought something around
100-150mA, charged in 7-10 minutes, so there is a good 2A left and the eventual voltage drop would be low.
I could use an LDO with a fixed voltage and current output, analog to question 3V Current source with low voltage drop. For this I found the MIC5205-5.0YM5-TR (http://www.farnell.com/datasheets/29605.pdf) but in the datasheet it says under 'Electrical Characteristics'
Vout = Vin + 1V so I guess that I can't use this one.
Other than an LDO I thought of a current limiting circuit with an MOSFET.
The circuit has 3 parts, first the voltage source of
5V with a maximum current of
Secondly, the current limiting part with the pMOSFET and the sense resistor, for this I thought of the PMN50EPE (http://www.farnell.com/datasheets/2621318.pdf) with
Vth = -2V typically. And lastly the supercaps in series with their internal series resistance and a voltage divider to keep the voltage across both supercaps equal.
Vin = 5V Id,max = 0,15A Vth = -2V Rsens = 1 Ohm Vgs = Rsens * Id - Vr1 Vth = -2V = Vgs,max = Id,max - Vr1 Vr1 = 2.15V R2/R1 = 2.15
The datasheet of the MOSFET says that
Rds(on),max = 0.07 Ohm, but in with lower current I guess this will be more? Is
Rds = 0.2 Ohm a more realistic estimate? Then the voltage drop would be
(Rsens + Rds) * Id,max = 1.3 Ohm * 0,15A = 0.2V.
I suppose this would drop as the supercaps are getting charged and
R4 are taking over.
Is this a good design? Can it improve? Like decreasing
Rsens and playing with the ratio of
R2/R1, replace with/add in parallel to
R3 and 'R4' 2 2.5V zener diodes. Will it fail completely as I oversaw something?
The datasheets of pMOSFET's give broad ranges for
Vth how do I know I have a 'good' one?
Some calculations to verify the mode of Q1.
Id = 0.05A -> Vsens = 0.05V -> Vgs = Vsens - Vr1 = 0.05V - 2.15V = -2.1V Vds = Id * Rds = -0.15A * 0.2 Ohm = -0.3V Vgs < Vth Vds > Vgs and Vds < Vgs - Vth = -0.1 -> saturation mode?
Id = 0.2A -> Vsens = 0.2V -> Vgs = -1.95V Vgs > Vth -> cutoff