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Setting a low charging current for LTC3225


I am using an LTC3225 for my application, which is only allowed to draw a current in the area of below 10 mA. Reading through the datasheet of the LTC3225, I couldn't find any hints for a minimum charging current (or a maximum value for RPROG), so I connected a 1.8 MOhms resistor to the RPROG pin, which should limit the loading current to 2 mA. However when I do that, the LTC3225 does not load with 2 mA (my application board as a whole only draws about 1.2 mA in this case), and when I changed the resistor to 360kOhms (hence setting the current to 10 mA), my board also only draws about 1.2 mA. When soldering a 36 kOhm resistor (corresponding to 100 mA), it works well.

What might be the problem?

  • The current mirror in the LTC3225 is designed for input currents in the range of 30-150mA.  Below 30mA the current-mirror current ratio is not 3000 as shown in the block diagram (datasheet, p. 6).  With a 36kΩ resistor at PROG the input current should be 100mA as you state.  You can experiment with RPROG to achieve the current limit you want, but you would want to qualify the use of the LTC3225 at that low current from part to part, over temperature, etc.