LT1512 slow charging

Hi, I'm developing dual power source device. 1st source is 12.4V adapter, the 2nd is 12v DELTA battery. PowerPath Controller is LTC4418. So the problem is that power consumption of LT1512 from supply more than 200mA and only 5mA goes to battery. Current is always about 5mA when battery fully discharged and also when battery is fully charged. With this issue because of low output current controller can't fully charge battery and charging time is very very sloooow. I charged my battery from 10.5v to 13.35v about 36 hours.Voltage limitter is set to 13,75V using R1 and R3 on my schematics and current is limited to 200mA using R5 with nominal 0,5 Ohm which was calculated using this formula: 100mV / 0,5Ohm = 200mA on page #7 of datasheet. Charging stops at 13.35V. But when I disconnect battery from the output, I see 13,75V on controller's output without load. I checked soldering on PCB 100 times, also I replaced all components twice except LT1512 because this controller is very expensive in Belarus (~$20). Also I tried to solder two 0,5Ohm resistors in parallel to increase charging current from 200mA to 400mA, but absolutelly nothing changed! Consumption from power supply is about 190-230mA and output of LT1512 is 5mA. And temperature of LT1512 is about 80C. Also I tried to charge battery to 100% (13,8v) using another charger and then I connected fully charged battery to LT1512. The result is: power consumption from power supply is about 12mA and 1-3mA delivers to battery from LT1512 to prevent discharge. So, the questtion is how to fix issue with low current output from LT1512? Thanx a lot.

L1 - Bourns SRF1260A-330M

R5 - Susumu RL1220S-R50-F

R6, R7 - Welwyn ULR15S-R005FT2

C2 - Wurth Elektronik 875075555003

C4, C5 - Panasonic EEEFK1E102AQ

All other capacitors are Murata X7R or NP0

All other resistors are Yageo 1%

  • 0
    •  Analog Employees 
    on Mar 11, 2020 4:18 PM 7 months ago

    I see a problem in the schematic.  The direction of the coupled inductor appears incorrect.  Compare the location of the dots on the inductor windings in your schematic to the schematic on p. 1 of the datasheet.  I think you need to exchange the connections on either L1A or L1B.

  • Thanx for response. But for this IC dot doesn't matter anything.

    1. Dot is important when using coupled inductor like transformer where the beggining of each windings matters.

    2. On page #7 in part "Application Information" you can find next: "... The two inductors shown are actually just two identical windings on one inductor core, although two separate inductors can be used." I suppose that for this application is more important that two inductors L1A and L1B should have maximal same inductance.

    3. On page #9 on Figure 4 you can see difference between connection points of L1A and L1B on "4.a" and "4.b".

    BUT! Tomorrow I'll try to replace my Bourns SRF1260 with two identical separate inductors. And I'm 99.99999% sure that this action will not solve my problem. But, if I'll be wrong I'll tell you thanx and sorry.

  • +1
    •  Analog Employees 
    on Mar 18, 2020 7:35 AM 7 months ago in reply to stanislavson

    There is LTspice model available, even with the demo board.  Please verify your design of component values and check on the expected output current, voltage, switching frequency, etc. 80C temperature on LT1512 at small power application sounds something is not right. It is a clue that the main switch inside is heating up abnormally.

  • Fil, thanx! I understood that about one week ago when I made some tests and experiments with LT1512... and I finally killed it. Now I'm waiting for new one. It seems that problem was in switch or one of opamp (Error Amplifier or COMP on Page #6, Figure 2. Block Diagram). Opamp Ifba worked correct. And YES, I used LTspice for my project and I tested schematics about 100...0 times before I began my art in Altium. Here is my .asc


  • As expected the reason was in broken LT1512. With new one now controller charging battery is it should.