I'm using the LTC3351 in a product for backup power in case of power failure.
I'm using 4 x 70F supercapacitors for the backups power.
My design is based on the evaluation board with soms alternations.
Input: 18 - 25 V, Max. 5 A.
Backup voltage: 16 V.
Backup Power: about 40 W maximum.
I'm using the schematic below:
The capacitors are on a seperate PCB. I'm connecting the supercapacitors PCB with a 12 cm wire assembly.
I have made and prototype two PCB designs. My problem is that the supercapacitors don't charge if they are below 7 V. If I charge them manually to about 7 V and then connect them to the LTC3351 board they charge okay and backup is working.
If the caps are below 7 V the LTC3351 on my board tries to start charging for about a couple of ms. After that it stops and does nothing.
What can be the problem?