Hi,

Here's an interesting issue I've been dealing with. So far I have been using LTSPICE to simulate this proposed circuit. This circuit involves a capacitor of 1.5 mF that should quickly charge in around 1 second. After 1 second, I would like to discharge the capacitor very slowly so as a switching regulator (boost converter) can see enough voltage (above it's minimum voltage). The switching regulator (a LTC3759 device) should then boost the voltage to 15 V and dump 1.2 Amps across a load at the output of the regulator. The time to discharge the capacitance voltage into the regulator to produce 15 V, 1.2 Amps should be around 100 ms. If I am constrained to have an input voltage to charge the 1.5 mF from 1.8 to 5 V, is there a way to produce 15 V, 1.2 Amps for 100 ms using the 1.5 mF capacitor. If so, would this capacitor need to be a super capacitor? If I need a super capacitor, would I require a super capacitor charger as well? Attached is my LTSPICE circuit so far. I had to use a bigger charging cap in order to discharge slowly for the regulator input to see above the minimum voltage (1.8 V). If I can lower the charging cap back to 1.5 mF while outputting 15 V, 1.2 Amps for 100 ms, then problem solved.Thanks!

 

Some basic conservation of energy calculations:

1.2A @ 15V for 1ooms is (1.2*15*0.1) = 1.8 Joules of energy.
To store 1.8J of energy at a (5V-1.8V) capacitance drop requires a capacitance of 351 mF (where E = 1/2 CV²).
Assuming the regulator is 85% efficient, this means you need a capacitance of at least 414 mF.
Thus 1.5mF is about a factor of 276 too small.

So you need a super capacitor of about 0.5F.
You can charge that from a 5V source through a resistor to limit the charging current to the rated peak value of the super capacitor.