# How to limit input current with LTC3625 and MPPT circuit?

How can I limit the input current with this circuit?

I need it to draw a max of 0.2A, so I can see how long it takes to charge different capacitors. Changing the value of R1 on PROG pin does not reduce the input current as per my previous question.

As you can see below, the input current is 5A. My real solar panel only puts out 0.2A, so I want to simulate how long it takes to charge different capacitor values before I build it. Also the simulator only seems to work with Voutset to 4.4v. If I make Vout=0, then it does not reach the output voltage.

## 1 Answer

The way to control DC to DC converters like this is through their feedback pin, in this case the PROG pin. (Usually linear sticks to 1.2V being the control voltage). If the voltage is too low then the regulator increases the length or time of switching. Too high and the converter decreases length or amount of switching.

The key is changing the feedback loop, topologies like this could convert this regulator into a CC\CV supply:

Precision CC CV circuit or power supply

However, this may affect stability. I'm not a power engineer by far. I did use a linear tech DC DC buck in a digital feedback configuration with a digital PID to change the buck from CV to CC mode for a battery charger with and MPPT tracker, and that worked pretty well.

Also the simulator only seems to work with Voutset to 4.4v. If I make Vout=0, then it does not reach the output voltage.

Check the value of the PROG pin, it should be around the value listed in the datasheet, if not usually they kick into some other mode.

• Thank you laptod2d. I checked the PROG voltage with LTspice and it does go from 1.2 to 2v, seems to be switching at a high frequency and yes that is the point of control for the charge current. Given i can easily control the charge current by using a resistor of 100K to 500K from PROG to ground, its the MPPT function that i really dont understand. Looks like this circuit is specifically designed for the specified solar panel, with has a peak power at 4.4V. All i really want is to charge a 2.7V 100F cap from a 5.5V 0.2A Solar Panel circuit.. cant find it anwhere Jun 12, 2019 at 10:16
• Well, after searching further, i found the LTC3355, which seems to do what i want. Variable input, with single super cap charger, and configurable Vout. Jun 12, 2019 at 10:50
• Any idea where to find a reference schematic for the LTC3355 in LTSpice so i dont have to creat it from scratch ? Much appreciated Jun 12, 2019 at 10:51
• @phantomrose1999 In the datasheet, if you like the answer upvote Jun 12, 2019 at 14:58
• @phantomrose1999 analog.com/media/en/technical-documentation/data-sheets/… Jun 12, 2019 at 14:58