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LT1512 - From 200mA to 20 mA without any reason

Hello

I took the LT1512 figure from the spec as attached. The battery is a 8,4V NimH 2000 mAH.

The current value is limited by R3 that is in my case 0.5 Ohm so the charging current is 200 mA.
R1 was defined further the spec formula, means 71k for R2 of 12,4k. Vin is 3V that seems coherent with the spec.

But it works only at the starting, several seconds and then the charging current goes down to 20 mA

I don't understand why ...

The battery is empty. 

When I take off and on again the battery, still 20 mA;

When I power down and up...200 mA during several seconds and then 20 mA;

Do somebody have some idea from where it could come from ?

Thanks a lot for your help

Vincent 

  • Based on your component selection I do not see an obvious problem with your circuit.

    I performed an LTspice simulation of your circuit, and the simulation runs OK.  Please see attached.  You can run the .asc model in LTspice.

    Things to check:

    • Is the capacitor (C2) polarized (electrolytic)?  A ceramic capacitor should be used.
    • What is the inductance of the individual inductor windings?  I used 33µH in LTspice.
    • Are the "dots" (winding orientation on the schematic) of the coupled inductor correctly aligned?  What is the manufacturer part number of the coupled inductor?
    • What is the battery voltage when you perform your testing?  
    • Is it possible to monitor the voltage at IFB and FB when you power up the circuit?  How does it compare to the simulation?
    • Could there be problems with the layout?  Figure 4 in the datasheet shows a good layout for this circuit.