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LTC3643 Boost mode current limitation

Hi everyone,

I am currently designing a power backup function using LTC3643.  I use four 470mF super capacitors in series but I had some bad issues. This function works perfectly if I use 6x 220mF in series instead of 4x 470mF.

Il would like to know how to limit the boost current to 1.5A max ?

Vin = 12.5V

Vcap = 18.28V

Vbackup = 11.5V

ILoad = 1.5A @12.5V

Here is the schematics :

  • Hello, The LTC3643 was not designed to charge a stack of SCAPs and does have limited ability to limit the charge current below the peak current limit. There is a way to set the maximum input current which reduces the charge current as the maximum input current is reached. This is intended to give the load the priority over the input current. The current limit, Load + input current to the charger, is limited using the R2906 to 50mV/13mΩ in this case, ~3.85A. As the load + charger input current approach 3.8V the charge current starts to reduce to help prevent the input current from exceeding the maximum input current. Please note that the input current limit does not prevent the load from exceeding the maximum current. It can only reduce the charge current to 0A. 

    Make sure you calculate the backup time with the total capacitor stack ESR if you have not already. 

  • Hello, thanks for your answer. I need to backup at 11.5V. And to get enough energy storage I need to have VCAP = 18.3V with a little form factor for the SCAPs. That is why I actually use stacked SCAPs because SCAPs with large capacitance and large voltage are to big to fit in my application.

    Otherwise, it is written in the LTC3643 specification on p12 :  "If I2 in the application is 4A, then a maximum of 1A of current can flow through the boost regulator. If I2 is minimal, then I1 will be limited by the maximum current of the boost regulator. Furthermore, if I2 exceeds the 5A limit, the ITH voltage of the regulator will get pulled low and the part will enter its sleep mode". 

    Does that mean the maximum current of the boost regulator is only driven by R2906 in my case ? Or Is there a way using ITH pin ?

  • Hello, I do not know of a good way to limit the boost current using the ITH at the moment. The only thing I can think of is a method that was used on the LTC3350 as described on page 22 of the datasheet where the current sense resistor is split to allow a lower current limit for the charger and higher current limit for load. 

    Something like that might be able to help for this case. The down fall is that it will have a IR loss in backup mode that might need to be accounted for. In this case a second resistor would be placed on the I1 leg.